These edge optimized optical solutions reinforce the portfolio synergies between our packet optical and fiber access solution sets and they drive more value to our customers adopting these combined solutions under a common suite of software tools. In summary, we continue to focus on capturing fiber footprint with our upgraded fiber access and optical transport platforms, while driving the adoption of our latest subscriber platforms, software solutions and high value services. While we remain confident in our long term outlook, we continue to see cautious spending from our service provider customers, driving us to take more cautious approach with our forecasted operating model. As a result, we will continue our focus on becoming an even more efficient and more profitable company with our best-in-class fiber networking portfolio.
With that, I will turn things over to Uli to provide a review of our financial results. And following Uli’s remarks, we will open it up for questions. Uli?
Uli Dopfer: Thank you, Tom, and hello everybody. I will cover our Q4 2023 preliminary results and provide our expectations for the first quarter of 2024. I will be referencing non-GAAP information with reconciliations to the most directly comparable GAAP financial measures presented in our press release and also certain revenue information by segment and category, which is available on our Investor Relations webpage at investors.adtran.com. In addition, we have updated the investor presentation to the site, which is available for download. Unless stated otherwise, all financials are presented in US dollars. Q4 2023 revenue came in at midpoint of our guidance at $225.5 million but down 37% year-over-year and down 17% quarter-over-quarter.
Our Network Solution segment accounted for 80% of revenues in Q4 2023 compared to 88.6% in Q4 2022 and 83.9% in Q3 2023. Our Services and Support segment contributed 20% of revenues in Q4 2023 compared to 11.4% in the year ago quarter and 16.1% in the previous quarter. Access and aggregation contributed 28.5% of revenue and was down 32.9% compared to the year ago quarter and down 32.1% compared to the previous quarter. Our optical networking solution category contributed 38.2% of revenues and was down 39.5% year-over-year and 26% quarter-over-quarter. Subscriber solutions was down 37.5% year-over-year but grew 22.3% quarter-over-quarter and contributed 33.4% of revenues. As Tom mentioned earlier, we continued to face a decline in service provider spending, driven by macroeconomic challenges and ongoing inventory adjustments.
International revenue made up 62% and domestic revenue contributed 38% of total Q4 revenue. We had one 10% or more of revenue customer in Q4. Q4 non-GAAP gross margin was 41.9% and increased by 277 basis points year-over-year and 155 basis points sequentially. The year-over-year and quarter-over-quarter increase is due to reductions in manufacturing and transportation cost and a more favorable customer and product mix. In the fourth quarter of 2023, we successfully achieved our target with a 15% sequential reduction in non-GAAP operating expenses. Q4 non-GAAP operating expenses were $97.6 million, decreasing by 15% quarter-over-quarter and 18% year-over-year. We reduced non-GAAP R&D spend by 18% and SG&A expenses by 11% quarter-over-quarter.
Non-GAAP operating loss was $3.2 million, which translates into a non-GAAP operating margin of negative 1.4% compared to negative 1.9% in Q3 2023. Our operating margin was at the upper end of our guidance range of between minus 7% and 0% of revenues. The sequential improvement in operating margin was attributable to higher gross margin and the successful implementation of our cost initiatives. The year-over-year decrease in operating profitability was due to the lower sales volume, partially offset by improved gross margins and operating expense reduction. The company’s GAAP and non-GAAP tax expenses for the fourth quarter of 2023 were $64.4 million and $73.1 million respectively. Given the current environment, the company decided to establish a valuation allowance related to our domestic deferred tax assets during the quarter.
Of course, the company will be able to release this valuation allowance as we return to profitability. Including the $81.6 million tax valuation adjustment, total non-GAAP loss was $82.9 million and the net loss of $85.9 million after adjusting for minority shareholder interest in ADTRAN Networks SE. This resulted in non-GAAP diluted loss per share attributable to the company of $1.09 per share. Turning to the balance sheet and cash flow statement. Cash and cash equivalents totaled $87.2 million at quarter end. Cash flow used for operations was $23.6 million compared to $6.8 million of operating cash flow generated in the previous quarter. The increased usage in cash flow from operations quarter-over-quarter was primarily driven by lower revenue inflows, partially offset by reduced expenses.
Trade accounts receivables was $216.4 million at quarter end, resulting in DSO of 88 days compared to 77 days in the prior quarter. Inventories were $362.3 million at the end of the fourth quarter, down $11.7 million compared to Q3 2023 and down $65.2 million compared to Q4 2022. Q4 inventory included a $3.3 million write off as we accelerated the end of life of certain products to streamline our product offerings. As Tom mentioned earlier, we remain focused on reducing our inventory levels moving forward. Accounts payable were $163 million, resulting in DPO of 67 days compared to 60 in the previous quarter. In summary, we are still experiencing cautious service provider spending due to economic uncertainty and continued customer inventory adjustments.
Given these uncertainties, we will continue to focus on aspects of our business that we can influence, such as managing our operational expenses and reducing our own inventory levels. We are convinced that the long term growth drivers for our business are fully intact. We expect that the investment in data driven infrastructure and the fiber everywhere future will continue supported by stimulus funding and the desire to reduce exposure to high risk vendors. We continue to focus on capturing fiber footprints with our upgraded fiber access and optical transport platforms, while driving the adoption of our latest subscriber platforms, software solutions and high value services. Consequently, for the first quarter of 2024, we expect revenue to range between $210 million and $240 million and we expect a non-GAAP operating margin between negative 7% and 0% of revenues.
