Yes, doubling the motor count (and gearing them together in a 1:1 ratio) will double the theoretical torque, but they will still rotate at the same speed.
Pretty close (I did the math in my head and came up with “about 1.5”). It’s always useful to figure out the target speed of your robot, given the game layout and your game goals. Most of the time, 3-4 feet per second is as fast as you need to go.
(Circumference of wheel in inches * Motor speed in revolutions per second) / 12 = feet per second
The circumference of the wheel is 2piR, or 10.21", the motor RPM is 96rpm, or 1.6 revolution per second. Doing the arithmetic, that’s 10.21 * 1.6 / 12 = 1.4 feet per second. There isn’t usually a need to go to five digits after the decimal in VEX math. 
If I were targeting 4fps I would go with 1:3 gearing on your drive train and high-speed internal gears. You could go to turbo gears, but your target speed then is not achievable with common gearing.
A picture of your robot would be really helpful, by the way. Good luck.
We have a 6 motor turbo base with 4" wheels. Going into this build we had heard all the “horror stories” of turbo drives so we did everything we could to make sure that it would not fail.
Step 1: Axles must be supported on 2 ends by either a bearing or a motor.
Step 2: Make sure the axle spins freely when it is not inserted into the motor. Everything on the axle should be kept in place with spacers, washers, and a shaft collar. Also, make sure that the wheel is free of any obstructions such as a chain or screw heads. We try to minimize weight and potential failure (loose shaft collars) by limiting one shaft collar per axle. We can get away with using only one shaft collar per axle because since we fill the rest of the axle up with spacers and washers. After we think we have the correct spacing, we spin the axle to make sure there is not too much friction. It should spin for 10-15 seconds after your hand spins it.
Summer prototype that shows an example of using only one shaft collar per axle:
Step 3: Make sure that all your motors are running smoothly. I highly recommend testing each individual motor and motor controller to make sure that they work as expected.
We did all of these steps and we still had the same problem. After about a 30-60 seconds of driving both sides would stop. We were confused for about an hour until we remembered that we had the same issue in SS where our both side of our base would stop even though only one side was visibly struggling. Originally we had 4 y-cabled motors and the other 2 motors in ports 2-5. On every cortex, ports 1-5 and 6-10 have their own breakers. Since the motors were drawing so much power at once they must have been tripping the breaker. We put one y cable in ports 6-10 and the other y cable on our power expander, and we left the last two motors between ports 1-5. This solved the problem for us without making any mechanical changes. Now, we can drive our robot around for a good 15 minutes without noticing any major speed differences.
To be more specific to your case, 4 turbo motors on 4" have a high chance of stalling – especially this season, if you are carrying around a MOGO with lots of cones on it. As others have said above, step down from turbo (240 rpm) to high-speed motors (160 RPM), or keep turbos and switch your wheels to a 3.25" diameter. Alternatively, you could keep your existing setup and add 2 more motors.
Also, I recommend watching this video made by a team that we are good friends with:
(Go to 2:06)
https://www.youtube.com/watch?v=uBb-wHCrfPQ
EDIT: sorry for the large image, I don’t know how to make it display in as a smaller size.
EDIT 2: Spelling
An “axle” supports a wheel, an “axel” is a figure-skating jump. Sorry to be the spelling police, but this is a pretty important robot part to get wrong. Your advice about drivetrain friction, on the other hand, is dead-on. Smart people should listen to you.
@Rick TYler Thank you for pointing my spelling mistake out!
Make sure all of your motors are in good health and also ensure that they work individually. Excess friction from a non-spinning motor is enough to burn out turbo motors.
Thank you for your hep everybody! We have decided that at least for now we are switching to high speed motors. Thank you again for all of your help.
This issue comes up every year as teams come to grips with the capabilities of the vex motors. To guesstimate a drive configuration based on the weight of your bot, you might use the spreadsheet I posted in this old thread.
One way you could tell if you’re overloading your motors (which I think you are) is to have someone hold the robot in the air, so it’s not touching the ground. If the wheels can move when you run auton or joystick control, then you know that the motors are working fine … but only up to the capacity they were designed for.
I also notice in the photo above that your omniwheel might be rubbing against the c-channel on the right (in the photo) or, on the left side rubbing against the standoffs or chain assembly. Be SURE that your wheels are not touching anything except the floor and the axle going through the center; rubbing will make your motors work extra-hard, no matter what kind of motors you have.
Are the shafts bent?