Is it possible to have surgery to correct a recessed knuckle (1/4") and shortened finger (1/4") on a mildly displaced 4th metacarpal that has fully healed?

It is possible: Yes, it is possible, but not necessary for cosmetic reasons. Most of the time when osteotomies are performed, it's a rotational malunion or if its so significantly shortened that you have weak grip strength. I think you may be disappointed if your reason you want it done is solely for cosmetics. There are a lot of risks with surgery and your motion could be diminished as a result of scarring.

Yes: But the real question is would you be functionally better after surgery considering potential complications of osteotomy, bone grafting, and internal fixation. If you already have good range of motion, no rotational deformity, and good strength--then you are likely better off without surgery.

Its possible to fix: Yes, its possible to improve the deformity surgically. The question is, are the risks of surgery worth the benefit to you? This surgery is called corrective osteotomy for malunion (re-breaking the bone). Hardware such as plates/screws are needed to keep the alignment. Possible risks are scarring, stiffness, infection, delayed healing, etc. This may not be worth it to you if your function is ok now.