I have a 1992 Rinker Captiva 236 3600 lbs with a Merc 454 and a Bravo 1 drive , I’m currently running a 21p Quicksilver Mirage at 4200rpm , I’m looking to get a little more top end and be at 4500-4600 rpm what would be my best bet?

Which 454 ?

Chevy

Okay thanks.

300,330,365,385 HP ?
Max RPM rating (flat tappet or roller)
Seems like you should be able to turn a 21 more than 4200. Are you sure boat doesn't need a tune-up?
3 blade?

Yeah...my 208 Baja with the 330 hp 454 would spin a labbed Mirage 21" to 5200 or more...no rev limiter on that engine, lol! Would spin a 23" Laser II to 4600 easily. With a 1.50:1 ratio outdrive. That was a 3400 pound boat. Sounds like something isn't quite right with your engine...maybe a little tired? How many hours? What ratio on the outdrive? I am guessing it's a 1.36:1 with that poor of performance...and if so, you'll want to drop down to a 19" Mirage Plus to start.

It’s 300 hp, I might of lost 200 rpm but I think that’s from scum on the bottom … engine just hit 600 hrs , just replaced the plugs, wires and also did a compression test.. all was good! I have no idea what ratio the drive…

There's a little decal on the outdrive, typically port side, that will say. Or you can (VERY CAREFULLY!) start the engine, put it in drive, then kill it, pull the distributor wire or some other positive means of making sure the engine can't start; have an observer watch the prop to see how many revolutions it makes as you turn the engine over one time by hand...a bit difficult but not impossible with a large wrench on the harmonic balancer pully bolt. You'll want to have some EXACT markings on both the engine and prop to evaluate the outdrive ratio.

Did you change cap and rotor also?
Check timing and make sure you are getting advance.
Did it run more RPM in the past or has it always been 4200?

Is the prop perfect?? A couple nicks or dings can make big difference. You could try a 19" Mirage Plus