Basically what FLAJill said but think of it this way; a top distance swimmer might go 39:00 for a 4000yds straight and would be sore the following day. The same swimmer might hold 3:50’s for 10x400yds on short rest like 4:15 and be ready to puke at the end.
For a distance swimmer (and I would argue for a triathlete who wants to swim better) the 10x400 is a better workout than a 4000 straight.