I would say it probably depends on gender and whether or not you intend to buy an adult or a baby. If you plan to purchase a baby I would start with something much smaller.

I don't think you'll have any issues housing an adult female in a 4 x 2 cage (many people here use the AP t8 of similar dimensions without issue)

The Herpstat is a quality product.

With the conditions where you are, It may be smart to forgo the idea of the RHP all together and use belly heat. Your preference.

If you do go the route of the RHP you could email Bob @ Pro Products and he will set you up with the proper equipment based on your needs and you will avoid any confusion about whether or not you're picking the right one.