Which of my sabers should i use as a support?

If it's Nero bride put her or Rama and after max levelling Saberlot put him

Artoria and Nero are both AOE, better for farming which is what most people would be doing right now, so I'd suggest either of those two, probably Nero since you have her levelled.

In my experience 4* servants are usually ignored. So I guess go with Artoria.

I had NP4 Saberlot in my slot for a while but no one used him. My lvl 90 Altera is not very popular either though.

People rarely borrow Saber, they borrow it but usually must be equipped by Kaleido and Imaginary Element also of course AOE for farm.
This is the popular refresh style by people, here you go.
Refresh Friend Supp, see Caster and Rider then pick what they need.
They often pick any rider with Monalisa Ce and Kaleidoscope and Waver with 2030, Buster Bond CE, FP+75, Kaleidoscope and Chaldea Lunchtime.

More often than not, people just grab whatever is at the top of the support list, AKA whatever is the highest level.

It's true that Saberlot is a great Saber, and he will probably be the one that you use the most once you get him leveled. However, he's not the type of servant that people usually borrow, so you probably won't really get any bites it you use him as a support.

People don't borrow sabers too often, but when they do, it's usually an AOE saber with Kaleidoscope. So if you have Kaleidoscope, you should go with Artoria. Otherwise, it doesn't really matter, so just go with whoever makes you happy.