Hearts of Iron 4 is an open question, because it's not out yet. But CK2, EU4 and HoI3 all function sufficiently well to play them on a Surface Pro 2, which is sort of '2013-2014 ultrabook' class specs. But like I said, they can be a little slow. If you can get something moderately more powerful on the CPU and GPU front, those should be pretty playable. Integrated graphics have come a long way over the last 5 years and the paradox games are more CPU than GPU heavy.
Desktop is the preferred way to play them, but there is obviously some convenience associated with the laptop or tablet/convertable form factor.
Surface Pro 2 specs:
i5 4200 CPU
HD4400 graphics
So try to beat that if you're looking for something portable.
Seem's the best option and just had a very quick look at those kind of specs for a lappy and it's a lot cheaper than expected so will look for better. Thank you for the advice and can't wait to be playing this kind of strategy again.