AFAIK originally it was said that you could have up to four different characters in ESO. In the betas we've been allowed to have up to eight. Will it still be only four in the final version, or has it been extended there too?

Don't think it was ever officially stated it would be four. Earlier betas had four slots, more recent ones have eight, and it's been officially confirmed there'll be eight.

https://help.elderscrollsonline.com/app/answers/detail/a_id/5710/kw/how%20many%20character%20slots

I just need one, but I would see 4-8 maybe 5 or 6 or 8... Crystal ball is kind of foggy.

It was just an assumption that there was a 4 cap, due to the first beta only having 4 slots. It's been confirmed that there is 8 now though.

I hope it's eight, I already have five planned.

8, i think they said it in one of the FAQ thinggie.