l was wondering if it would help or hurt the performence of a wet suit if l could get some under armor to make my 4/3 work in a little better colder water.

it depends on the person you are, but it might be harder for the wetsuit to work

It will not make you any warmer, however it will help prevent you from getting a rash.

The Surfboard Man
http://thesurfboardman.com/

It will interfere with the flow of water in, out an through the suit. It will become quickly waterlogged and weigh you down. Salt water weighs well over 8 pounds per gallon. If you wear anything under your wetsuit, wear a rash guard.

Last week l tried a pair of thin thermal pants under my wetsuit, and it did weigh me down a bit. l am not sure if it kept me any warmer.

Under Armour is basically just a tight fitting rash guard. Rash guards do not provide warmth unless they have neoprene panels. You can where something like a wetsuit vest or top under your wetsuit for added warmth. In fact many of the wetsuit companies make neoprene vest with attached hoods for this purpose.