The only way to do this is cancel the reservation for 4 and rebook it for 6.
You could try just going in with two extra people, but going from 4 to 6 means you need a whole new table size, and you don't say where the ADR is but if it's an ultra-popular and small restaurant like Le Cellier or CRT, I think it's likely that they won't be able to accommodate the extra people.