For one it wouldn't even work, delegates wouldn't shift to RP because their king has accepted a bribe.
Second, that's what Romney would (will?) try to do to get his endorsement, to Romney $4M is nothing and he might even give him more cash.
Third, politicians should (and on occassion do) go to jail for getting paid to endorse someone.
And in addition, voters should (but sadly will not) be intolerable of such practice.