From the report above.
"....Going forward, an area of potential concern we identified is the apparent planned concurrency between Block 4 development and the procurement of Block 4 aircraft. Our assessment of DOD’s most recent Block 4 schedule (from August 2016) indicates that DOD was planning to request funding in February 2018 to purchase the first aircraft with the initial increment of Block 4 capabilities. This will come as part of DOD’s fiscal year 2019 budget request, more than 2 years before the development and testing of the first increment is complete. In addition, program and officials acknowledged that the aircraft’s current data processor is operating at maximum capacity, and an updated processor with increased capacity is likely needed for the first increment of Block 4 to function as intended. F-35 program officials acknowledge this risk and note that an updated processor may not be available until the second increment of Block 4. This poses a risk that the testing and delivery of the first increment of Block 4 capability may not be achievable as planned. As a result, DOD may be negotiating prices for those aircraft without knowing if or when the more advanced capabilities will be delivered and whether they will function as required. Consequently, Congress will be faced with the challenge of making funding decisions with limited information. Figure 1 illustrates this concern.
According to our past work, programs that limit concurrency between development and procurement are more likely to deliver promised capabilities within estimated cost and schedule parameters. Concurrency has been a significant contributor to many of the problems with the baseline F-35 acquisition program, which is currently expected to cost more than $104 billion, or 45 percent, over the original estimate, and to be more than 5 years behind schedule. As of April 2016, DOD estimates a total of nearly $2 billion in concurrency costs in the baseline program.
We are not making any recommendations at this time because program officials told us that the concurrency issue is being considered as part of their reassessment of Block 4. We will therefore reassess this issue when DOD’s F-35 Block 4 baseline report is issued and brief the congressional defense committees on our findings."........