Well, let's look at the release dates of their last four firmwares for a general idea.
3.80 - December 17, '07
3.90 - January 30, '08
3.93 - March 18, '08
3.95 - April 8, '08
The jumps between them are ~45 days, ~48 days and ~20 days respectively. So on average, there's 38 days between updates. Let's just assume Sony will be sticking to that - it won't be exact, but let's just say it's close enough. There are four possible updates between now and the 4.00 release - 3.96, 3.97, 3.98 and 3.99. Assuming Sony won't leapfrog over any of those, 3.99 will be released in 152 days - approximately 5 months, sometime in September. But I think Sony will ignore at least one, bringing that date back to July/August.
So, let's say the last firmware in the 3.XX series comes out in August. How long will it be before the next major release - 4.00? Let's look at how long it took them to do similar jumps.
1.52 - June 15, '05
2.00 - July 26, '05 (JP), August 24, '05 (NA/EU)
NOTE - 2.00 was the only major release that was released earlier in one region than another. So I'll use the JP date.
Difference - ~41 days (staying roughly in the 38 days I said earlier)
2.82 - October 26, '06
3.00 - November 20, '06
Difference - ~24 days
So, all updates are staggered equally. So therefore, going by my prediction of the last 3.XX series release in August, I predict 4.00 will be released in late August/September.
