How much hp and torque can SSM#4 handle.

I would like to add to this question.

Is a #4 shaped like a #5 with the gears and bearings of a #3?

Yes the #4 is shaped like the #5, don´t know about the gears and bearings.

I believe the #4 where rated at Merc at 750 hp max ,and 750 lbs /ft
But we had a # 4 with a 1000 hp twin turbo ,and no faillures .

I had #5's on my 04 43 Nortech. I was told the #5's were good for 750-850 hp and 850/tq and the #6's were good for 1000 hp and 1200tq. Eddie Young built some 750 hp 760 tq 600 C.I. nat. asp. motors in that 04 and those 5's to this day have yet to have a problem. It was a great set up all the way around. Are the #6's better yes, but a hole lot more coin. Jeff

The biggest problem that I see is getting parts if one breaks.

Out of curosity what is the difference between SSM #3's, #4's #5's, and #6's? Disadvantages, etc.? Which ones to stay away from when buying a used boat?

Big differance, #3s`& #4s came with large and small prop shafts . The # 4s can handle handle more power because of the larger internal shafts. I ran well over 1000 hp on 3s only problems was with the small prop shafts. Installed large prop shafts and they held up very well.
#5s have larger bearings,gears and a stronger gimbal.Also 4s & #5s have a much larger skeg for better handling on the higher X .
No comparison between s the 6s and 3s,4s,5s, plus the steering and trim are built in the gimbal.Old wet sumps are the best bang for the buck!!!

If price is no problem 6s will carry much more resale later on. The parts for the rest will only get harder to find in the future, which will make them cost more than new 6 parts. just m 2Cents. :d

Dry Sump Six Drive
Features
Swept-back skeg design.
Innovative dry-sump lubrication system uses one-fifth the quantity of gear oil used in previous models.
Comes standard with the 850 SCi and 1075 SCi. Available as an option with the 525 EFI and 600 SCi engine packages.
Upper gear housing allows for quick change of gear ratios.
Large, heavy-duty universal joint and driveshaft.
Forged lower-unit gears provide greater strength.
Input rated up to 1160 foot-pounds of torque.
Hydraulic power steering cylinders are designed into the drive unit.