I just watched the 1st place 1-4 run and I noticed that he had 200 hp. Is there a specific difficulty you have to play on? (I always see people play on violent which is the main reason I made those post)

there are no rules stating that you have to play on certain difficulty. the only rule being that you cannot switch between difficulties during the run.

correction: Doesn't say anything about swicthing diffculties in the 1-4 level rules opps.

Violent is typically used because it makes the enemies move faster (therefore running to their death faster). There are some cases, like in boss levels, where you have an instakill of some sort that invalidates the need for them to move faster. Therefore, since stamina regenerates faster on harmless, that is sometimes used.

There is no rule stipulating that you have to run on either, with the exception of multi-level runs that must be done entirely on a singular difficulty.