I have a 90A main service and a 200A sub with 200A bus. See pictures attached.
I currently have a 3.9kw solar system on a dual 30A breaker.
How much more solar would I be able to add?
For back-feeding solar into a panel breaker, the limit is: Panel Rating * 1.2 - Main Breaker. Assuming your main breaker is 90A, that's 200A * 1.2 - 90A = 150A solar breaker, maximum.
You can't plan to run breakers at 100% continuously, you must derate 20% so your theoretical maximum on this panel would be 120A of solar or 28.8kW at 240V.
Your power company and local laws will probably set a far lower maximum system size, especially if you are allowed to take advantage of net metering. For example, here in MD with BGE, the maximum system size is capped such that the total annual projected production does not exceed the average annual consumption. My house with 400A (CL320) service was allowed only about 17kW of solar based on our consumption.