Which Sealant/Caulk to use??

4000 is a fast dry formulation and will skin over quick in the hot sun but it sounds more like you had a batch partially cured in the tube, not totally unheard of. Was the "born on" date current? It usually behaves very much like fast cure 4200 or 5200 as far as skin times go. I've had to resort to checking dates when my wholesalers deliver product lately, they're not moving stock like they used to and a lot is expired when they finally send it out.

Are there any updates on this? I've got weathered caulking topside, where wheelhouse sides meet deck (black lines in picture). Both are gel coated surfaces. Also the caulking around wheelhouse windows. This time, I want to use white.

So is it the 4200, 4000UV, Sikaflex 295 (UV), Lifeseal, or try and find a DOW product?

I like lifeseal myself and use it a lot when I am looking for less than 5200 bonding strength.