Yup, I personally use the Proficiency bonus (called Adventuring Bonus, to avoid being confused for the weapon proficiency bonus from 4e) from 5e instead. It make success a little easier for the first few levels, but after a time, it helps to avoid the bonus treadmill. Same with the +X weapons.
I usually decrease all Defenses by 2 for the monsters and half-hp. Makes the combat faster.
I think it wouldn't be too hard to mechanically convert a 4e class into a 5e class or vice versa, or convert a 4e monster into a 5e monster.
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First problem is that HP don't scale the same, and 4e character lacks HD.
Second problem is that damage scales a bit differently in 4e than 5e.
Third problem is that attributes scale from 1 to 30 more than they do in 5e from 1 to 20.
Forth problem is "attack vs fortitude" vs "force con save", and FRW defences vs attribute saves.
Fifth problem is magic item scaling differences, and attunement vs daily powers+christmas tree.
Sixth problem is 1/2 level bonus vs proficiency bonus.
Seventh problem is the +2-3 attack bonus on weapons.
Eighth problem is the OA/Reaction mechanics and expected OA triggers are different.
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None of those look really hard to work around.
The characters will play a little bit differently than 5e characters would; for example, the Ranger wouldn't suck.
Similarly, mechanically converting 4e monsters into 5e monsters doesn't look hard. You'd start with stripping out level scaling on a bunch of stuff.