My wife has chronically cold feet so she's asked for boot heaters for her birthday.* That she'd want ski gear at all is a pretty exciting for me, so I'd like to get her something that works. Are Hotronic E4s my best bet? Are they worth the premium over the E3s or older M4s still floating around? Are there other brands that I don't know about that are as good/better/better for the price? This is an area of ski equipment that I know very little about.
*This isn't something that only happens when she's in ski boots, so I don't think it's a bootfit issue.