You have to cut them apart - the flats are made to separate - and remove the ones that have sprouted. It is best to never mix varieties in the same container.

Dave

I confess that I do not separate them out when this happens. I take them off the heating mat and uncover them. The rest sprout pretty quickly after. You are (were) only at day 4. If that were the case after two weeks, I might do things differently (maybe not). Even seeds of the same variety can vary in the time they take to germinate.

Maybe I'm just lazy, but this works for me. The less I fuss with my seeds, the better they seem to do.