Theoretically, best choice would be an order to revote the 4 states, this time according to law.
But SCOTUS may throw it back to the state legislatures to decide who the proper electors are. Since that is the remedy in the Constitution for a failed election.
OR, ....they may simply strongly recommend serious electoral reforms for the future, and hand it to Biden along with a scolding of Democrat election officials.
Another poster said that SCOTUS could simply say that the results of the elections in those states were invalid. Neither candidate would have 270 since the slates of electors from those states would be in dispute.
Constitution kicks in at that point, and the matter goes to the House.
Each STATE, represents 1 vote. If the state has more R Reps than D, then it’s an R vote. Vice versa. Currently, there are 26 R delegations, and 23 Ds (and 1 I, which could not caucus with the Ds). DJT remains POTUS.