Overall, those are good scores for a not-quite-four-year-old. Do you feel that the working memory (which is quite low) is consistent with anything you see IRL? I notice you say he has some attention issues. That can certainly affect working memory scores. Were the subtests within the indices consistent with each other? Or was there diversity within clusters (roughly > 3 scaled score difference)?
We didn't have quite the same situation, as we didn't have a gifted program option. We ended up skipping K and going from 4-yo preschool straight to 1st. Similarly, though, we had a very excitable, impulsive preschooler, with fleeting attention, and had to consider what would provide the best match on the maximum number of indicators. We considered social, cognitive, and physical factors (e.g., naps, length of day), among other things, and ultimately concluded that, for our child, the mismatch in social maturity was not going to be fully closed anyway, by staying with the younger children, and the increase in social mismatch involved in moving into the higher grade was smaller than the decrease in cognitive/academic mismatch.
In your case, I would ask what the classes are actually doing in the standard K vs the gifted preK. It may be that the pace and quality of instruction will help make your decision clearer. It may also be worthwhile sitting with him over a period of several encounters to try to tease out what it is he doesn't like about school.
