You know how many Herpstat 4 tstats Ive seen on this forum fail? None.
You know how many Accurite combo gauges I have had that have failed? 4
The Accurite units are great when they work but every one I have bought has died.
I have two Herpstat 1s, a Herpstat 2, and a Herpstat 4. I have never had a single issue with them so I say if you want the 4 and know you could use it for future snakes, get it. If you are concerned about having a back up tstat I would buy the Herpstat 4 and then keep an eye out for a spare that someone is selling used. You can find them here, Fauna, and Kingsnake. I bought two of mine off of Kingsnake and got a great deal.
I would hold off on the humidity cable until you are sure you will need a fogger. I really doubt you will.