Welcome to our forum.
On a side note, how do you sit in a hot tub for 2 hours?
I did inquire about this a few years ago for a customer who used their Grandee for extended soaks in the winter and the end result was you are stuck with the 4kw heater.
Two hours with the cover open will result in heat loss which is normal. When you are in the spa with it running you are making the heater work harder to come up to temperature because it is now battling ambient temperature.... The heater is more then adequate even in the cold winters of NY. My suggestion would be to make sure you have the temperature set higher before you get into it. It will rise in temperature much faster with the cover closed and when you are in it the spa should maintain temperature just like it maintained the 99 degrees you had it set at when you got in it. Just my two cents..
Finally regarding why Grandees are sold with inadequate heaters, my only guess is that maybe HotSpring doesn't want to scare people away with a tub that draws so much power and needs a 60 amp breaker and #6 gauge wire - which I needed between the main electric panel and the sub panel.