I'm wondering if it will be a lot of work to fit an f4 engine to f3

 

simple answer: yes.

 

And the long answer of why it's hard?
I'm assuming we're talking carb model?

 

the difference in f2 and f3 is almost minimal,
i hear tons of people swapping parts, including engines.
those are almost identical steel frame bikes.

the f4 might as well be a kawisaki bike in comparison.
aluminum frame, different build, different carbs.

from f2benny:
OK, in a nutshell:
All from memory so don't bust *****.
F2. Great all arounder. About 90 HP. Steel frame 160 rear tire. 91-94. Minor upgrades to fork and rear shock in 94 . Aluminum rear pegs and remote shock reservoir.

F3. Still excellent all arounder. Better suspension than pre-94 F2. More like 95 HP. Added ram air. Same motor as Honda 599 now. Still steel frame. Updated in 97 year. Still 160 size rear.

F4. Aluminum frame and about 100 HP. More sporty than F3, but still decent all arounder. Carbs. 180 rear tire. Ran 99-00 years. Yellow and red in 1999. Silver and Yellow in 2000. Some oranges too in 2000 if I recall.

F4i. 2001-present. FI added and two part seat 2001-2003, then back to banana seat. More sporty than F4, higher seat. Less comfortable. Much worse fuel ecomomy. Added dual H7 headlights.

RR. Race bike with radical seat position compared to others. Weak for passenger comfort too. Underseat exhaust.

 

can it be done? yep
pain in the *** welding custom mounts? yep.
will it cost more time/money than to hunt down and build on an f4 frame? yep.
would be it fun? yep
would you probably end up throwing tools around like baseballs in your carport? yep.