3/4" EMT has a cross sectional area of .5330 in². You are permitted to fill that 40%, or .2132 in² in your case.
Each #6 THHN conductor has a cross sectional area of .0507 in², and four of them total .2028 in².
You're very near the fill limit, but in compliance none the less.
I'm a bit curious why four #6's? If this is two hots, a neutral, and a ground; the ground is permitted to be sized at #10 to accompany #6 hots. If this is three phase, with three hots and a neutral, and you're using the EMT for the ground path, disregard my concern.
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