Which one of the 4* servants should I pick?

Even if you are going to pull for Scathach, Elisabeth is a nice addition to your team due to having synergy with Jalter. But then again, pick first who you like.

EMIYA will be incredibly good in the future after his strengthening quests and interlude. For now, he's decent at best.

Heracles is an obvious good choice and even if you have a lvl 100 Cu Alter, the best berserker right now, Heracles can fulfil a better survivalist/clutch role after you get his lvl 10 bond CE.

Marie can be a good staller with incredible survival skills thanks to her 2nd skill and heals., although she might not work well with crit-based DPS because of her rider star weight.

Elizabeth is a decent support and can synergise well with J'Alter, but her supportive capabilities are limited to only boosting damage and she herself can't deal a lot of damage.