Yeah, you can always level up other Servants later. I mean eventually when you clear the story and stuff there's downtime between events, most people spend that time either farming materials or XP to create a backlog of Servants. I have a bunch of Servants maxed in the JP version that are mostly useless that I just leveled because I liked them (Medusa, for example).
Saber Lily is iffy. Despite being 4*, she more like a 3* with a golden paint. I'd say if you like Saber or don't have any other option, sure, you can use her. There's never going to be a situation where you will need her honestly - if you really need a Saber for a fight, you can just use someone else's Saber. But it's up to you. I personally won't bother leveling her, at least not at first.