4 Mini CIMs would probably be very noticeably weaker on the drivetrain than 4 CIMs.
According to the VexPro specs, the Mini CIMs have 66% of the power and 50% of the torque (when adjusted for their different free speed rpms) of the standard CIM. As an educated guess, the torque is probably a more important factor for acceleration and pushing than the max power is... So the 4 Mini CIMs would be pretty similar to a 2 CIM drive. In a nutshell, 2 CIMs is "inadequate" for a ~100+ pound that drives on an ordinary surface and will be accelerating beyond 10fps, pushing robots around,
or driving over field obstacles. Given that a few teams are even using 6 CIMs now, I'd say you're best off with 2 CIMs & 2 Mini CIMs or 4 CIMS. The extra .5# per CIM
on the drivetrain is well worth it for >95% of FRC robots.
As some anecdotal stuff: 1519's used 2 CIMs on three of our competition robots... one was Speedracer, a fast lapbot from 2008; the other was Fezzik, our hurdling robot from 2008; and one was Colonel York, our 2009 robot.
Speedracer from 2008: was a ~40# robot that had 2 CIMs driving the rear wheels and ackerman steering in the front. It was geared fast, but not excessively so. It accelerated well and never had overheating issues. Definitely no regrets on using only 2 CIMs. We have
video of it on our team webpage (our more recent videos are on youtube, though).
Fezzik, our hurdler from 2008: was initially designed to meet all robot rules (weight, volume, interchangeable electronics, bumpers) together with Speed Racer... so we took a couple weight-saving compromises... one was to use only 2 CIMs. It experienced significant over-heating issues after matches - in elims they got hot enough that you couldn't keep your hand on them! After our first event (and the interchangeable robot thing was ruled illegal -
see thread about it) we put the extra 2 CIMs on there. It was probably was ~11-14fps... moderately fast but nothing noteworthy.
Colonel York, our 2009 robot: ran on the low-traction surface using the required slick wheels... as a result there was no need for more than 2 CIMs of torque. I'd venture you could've used 1 RS550 on each side of a tank drive that year, given that the wheels had some <0.1 coefficient of friction on the field surface.
Every other year we've used 4 CIMs in either single-speed or two-speed transmissions. Again, if you really need the weight maybe switching 2 of the CIMs for MiniCIMs would be worthwhile; however, I'd venture for most robots there are better weight saving measures.