Is it possible to control four DC motors with Raspberry Pi at the same time? If so, where can I find info on how can this be done (from the physical aspect)?
Yes, it is straightforward.
You need a motor driver board. Each motor driver board will typically drive two DC motors so for four motors you would probably need a couple of motor driver boards.
If we assume these are small hobby motors they will only draw an amp or so at 6V. If that is the case look on eBay for L298N or L9110S based driver boards. L9110S based ones can't supply as much power as a L298N based one but they are much simpler to use. You should be able to buy the boards for a few GBP each.
To control each DC motor (in direction and speed) will require either 2 or 3 gpios. So for 4 motors you will need between 8 and 12 spare gpios. Recent Pis have 26 available gpios so this shouldn't be a problem.
There must be thousands of tutorials showing how to use these boards. Google will find them for you.