Your question indicates that you don't.
If you did you would realise that 4 tanks in a group doesn't mean 4 people all with maxed out vengeance (which is the source of a tank's high DPS). Your vengeance and resulting DPS is as high as it is because you're the only one getting hit, with more tanks either you would still be getting hit as much giving you the same vengeance and them none (i.e. your DPS would be high but theirs would be lower than the DPS you wanted to replace with more tanks), or they'd be the ones taking the hits which would leave you vengeance-less and with less DPS. Either way your idea doesn't make 4 tanks better than 1 tank + 3 DPS.