my old sub panel has 3 single strand 4awg which is ran to my meter loop on a 50amp breaker. Someone had cut a leg on both ends so now I can only get 120. Its only a 4 space sub. When I tried to add a third breaker for some reason it wouldn't work. also how do I wire in the new wires in the new sub box from the main?
Yes #6 wire would be fine on a 50 amp breaker. Today’s code requires 4 wire and you will need a grounding electrode at the the shop. I would verify that the original #4 is not serviceable prior to spending the $ on a new UF cable or putting in conduit and wires, over the years I have found several feeders like yours that were cut because the owner was afraid of 220/240v. Most don’t realize the voltage to ground is only 120v there is only 220/240 between the 2 hot’s. I had a friend do the same because he did not understand and realized his welder would not work and asked me to fix it, prior to that I wondered why someone would do this (I have seen it about 5 times) where there was no damage to the feeder.