I would keep with 3s honestly. In anything but a tiny 1/18th you will be pushing it really hard on 4s. If you want 4s you should step up to a larger ESC, the m25 just gets hot on that high of a voltage.
4s may be too much for the internal bec, so 3s may be a better choice. Unless you want to run a ccbec, but that seems overkill in a 1/16 buggy. Unless you have the batteries laying around. That hacker motor is nice, I had one in a minit for a while and ran on 3s!