what is321 base 4 + 123 base 4 equal?
You can either add the base-4 numbers, remembering the "carry" any time you have a value of "4" or more (just as you "carry" in regular decimal arithmetic when you have a value of "10" or more), or else you can convert the numbers to decimal form (that is, to regular base-10 numbers), do the addition, and then convert back.
I don't know what you've learned, or what your preference might be. Please reply showing what you've tried so far. If you're needing a review first, please study this number-bases lesson