Should i prioritize a really good 1-3 star servant over a bad 4 star?

Robin and Astolfo are different servants, astolfo is AoE Rider and Robin ST NP archer, Robin is a Boss killer and Astolfo is for clear waves.

Personally if I must to chose between both I will go for Robin because I realy don't like Astolfo and for AoE raider I chosed Medusa (waifu), both are 3*.

All depend what other servant you have to fullfill those roles.

Usually yes, if they fullfil the same role. Robin and Astolfo have different roles, so it's better to analyze your team and see what you lack. If you have both roles with good servants, then level up the one you like the most.