Max size Conti do the 4000s in 650c is 23mm which is too narrow.
Ah, good point. Mind you, for tiny people 23mm is enoooooormous. 23mm tyres actually look pretty fat on a 650c wheel I reckon. But if you want wider, then I'd agree 26" is a better choice.
Isen AllSeason can be used with either. I'd go for 650c and GP4000SIIs, assuming that Conti carry on making 650c tyres when they switch to GP5000.