This one is pretty straight forward.
In your pawn/character, add a camera and set the Constrain Aspect Ratio tick box to true and then the Aspect Ratio to 1.33.
You camera will now be locked to a 4:3 Ratio as seen below:
If you need to lock it to a 16:9 Ratio just use the following value 1.78 instead of 1.33 for the ratio value.