I have a 5” FPV I built back in 2018, I never really used it as it scared the Sh*t out of me! I have recently retired and have much more time to learn how to fly it now! It has a Bardwell F4 AI0 FC running BF 3.3.1.
I tried to update to the latest BF and the throttle is erratic, in the motors section of BF configurator when I increase all 4 motors ramp up and will not come down when I decrease the throttle. All the other 3 stick motions operate correctly. everything works as it should in the receiver section. Rolling back to 3.5.1 and all works as it should? Any idea what is going on?

Which version of the board? AFAICR the last was V2.15??

IIRC the early versions were right at the very start of F4 capabilities, so more a F3 with tweaks.

Earlier hardware will likely have issues with current BF versions that are more angled towards F7's and/or recent F4's.

My suggestion is to stay with a BF variant that was OK on F3 boards, which effectively you have done.

It is an older board, so I like your upgraded F3 idea, however lots of people seem to use BF 4.x on F3 boards. So I kept researching, I think I have found the problem, the default configuration for BF 4.x loads up with air mode permanently enabled, I think this is what was causing the throttle problems. I have not flown yet but disabling air mode has made everything behave the same in BF 4.1.0 as it did in 3.5.6 so I am hopeful!!

Air Mode is used to enable the flight controller to still have some authority over the motors at zero throttle so it can keep your quad in the air. Without it, if you drop the throttle to zero while doing acro manoeuvres (which is a common thing to) your quad can start to tumble out of control until the throttle is raised again. So just keep that in mind. If you never do acrobatic flying and/or never drop the throttle to zero, then having Air Mode switched off won't be a problem.