5500 H&W file a Final with 4R?

Company is sold to another larger company which brings all employees onto their set of benefit plans.

Employees are covered by selling company for 2 months after the purchase date so as to finish the year off at 12/31 with the same benefits.

At 1/1 they are under the new company's benefits.

1 employee stays on the old plan for two more months (ie the plan exists until 2/28).

No need to file an additional 5500 since beginning of final year < 100.

So seems logical to file the 12/31 5500 with Code 4R and then mark as Final?

I would only mark the 12/31 5500 with Code 4R, and NOT mark as final.  Typically, when you file a final Form 5500, the number of participants at the end of the plan year is zero (i.e., the plan is gone).  In this case, end of year participants are "1."  The DOL will know by the use of Code 4R alone that a Form 5500 will not be required for the next plan year.