They can both do quite well with that spacing; keep in mind that citrus can be pruned to fit its space. The only caveat about pruning citrus is not to prune them up to expose the trunk to the sun; the trunks can sunburn which usually kills the tree.

Personally, I would amend the whole 4 ft. wide area with some compost, maybe some perlite, and maybe a little peat.
Before you do that dig a hole about 2 feet deep and fill it with water; if the water does not drain out in 24 hours, then your amended soil will be basically sitting in a big clay "pot"; so it would be better to plant in unamended soil.