One word - PWM - ok, three words...Pulse Width Modulation.
Keep it simple, use a PIC10Fxxx or something very simple. Use 2 of the outputs to run the motors, use 2 of the inputs to control the speed and/or direction of each motor. Write a simple program to use Timer0 to set the frequency of the PWM, use the 2 outputs going to the motors to turn on a couple of N-channel MOSFETs. Control the duty cycle of the pulse width with 2 buttons and a little ingenious programming. Do a Google search. You'd be amazed what you'll find.
If I've lost you on that part, you probably need to do some reading up on how regulators work, and for that matter basic electronics.
11.7v @ .7a = 8.19 watts, that's what you're drawing at the input.
4v @ .7a = 2.8 watts, that's what you're drawing at the output.
Where is the extra 5.39 watts going? Heat...all heat. Look at the datasheet for the LM317. You'll see that you can't possibly dissipate 5+ watts without a HUGE heatsink.
If you really must stick with a couple of LM317's, try preregulating that 11.7v down to 6v first, then re-regulate that down to 4v.
And for godsake, put some heatsinks on them regulators. You're killing them!!!
(climbing on soap box)
$20 says this is some sort of 'graduate' project...and it's gotta be done soon!!
(climbing off soap box)
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