Are all four braces at the same elevation?

Two coming in with a through mortise has been done many times.

Four coming in at the same elevation doesn't leave much post left at that level.
It would be better to make two either higher or lower so that they don't all join at the same place.

What thickness braces are you thinking about using? With 7x7's you could use 3x5's and do 1 1/2" tenons.

Jim Rogers