Could just be a hot chip, they do exist! Without a delid the only options for temp would be more aggressive fan profile, better paste (unless already good), delid or drop clock. Yet to play with 9900k myself but from what o can gather they're pretty toasty chips out of the box.
If 4.9 drops vcore a good amount, temps should come way down and that extra 100mhz on core you loose won't even be noticed apart from benching.
Everyone seems to be hung up on numbers these days. I've got an 8700k that'll run 5.4g for daily at 1.4v but keep it chugging away at 5g for a lot less vcore. Don't notice the difference one bit when gaming. (1440p+)