There's $17B set aside for future fighters. For some years the purchase of 28 additional F-35A options has been coupled in announcements with words like "or a future capability" I take that to mean more F-35, or more of something else. I'll let someone without a cracked crystal ball decide which.
As for Block IV, I'd be astonished if we don't stay in lockstep. These aircraft need to stay relevant for decades, and falling behind right at the start would be a monumental planning failure
oldsig
Mate, that figure of $17b you've mention is for the 72 F-35A
currently being procured, nothing to do with the potential additional 28 aircraft.
To break it down a bit more (and these figures quoted are from the Senate Report on the F-35 back in late 2016), at that time the program budget allowance was $17.1b, which broke down into $2.6b for contingency funding, and $14.5b for the 72 x F-35A, support systems, training, weapons, and infrastructure, but not sustainment costs. But in that report I couldn't find any figure mentioned for those additional 28 airframes.
The 2016 DIIP had slightly different figures, $15.3b for 72 airframes, $1.4b for facilities at Tindal and Townsville, approx. $360m for redevelopment work at Williamtown and Tindal (still roughly around that $17b figure).
There is was a
separate budget allowance of between $6b-$7b called,
Air Combat Capability - Fourth Squadron.
The new 2020 update has slightly different figures again,
F-35 Lightening II $9.9b-$17b and
Additional Air Combat Capability $4.5b-$6.7b
In a nutshell, there is a budget allowance for the current 72 airframes being procured and there is a separate budget allowance for that possible 4th Squadron, etc.
Cheers,