Hi I have anew 4ft wooden arch south facing,
what are the longest flowering climbers I can put in
Not enough info there to make valid suggestions.
I assume you mean 4 feet wide? What sort of depth is it? How is it located in relation to other planting?
Lots of clematis will suit, and I expect roses, but it'll depend on what room there is, what else is round about the arch, and also whether they're going in the ground or in pots.
Most people will plant something at each side to give a succession of flowers through the season. One for spring and one for summer to start with. If the arch is deep enough [ie front to back] you can potentially have more.
It's a place where beautiful isn't enough of a word....
I live in west central Scotland - not where that photo is...