4* servants in stock.

Atlanta and Anne.are probably not quite necessary for investigating. But Atlanta's 1st skill is very valuable you can consider to have high-level for one-turn punch.
dEon is one top taunt unit, excellent for many challenge quest. At least have her to lv70.
Sumanai is great after strengthening. And you are lacking of AOE sabers. So yes.
Marie is much durable than any other riders. I dont think Medusa would out-performance than her.
Helena is even more compatible with Tamamo than Liz. off course, her NP damage is not comparable since Liz is NP5, and you already have Nito.
EMIYA is same as dEon, excellent for hard quests. His 3rd skill is unique and deserved deeply investing (skills). But you have a bunch of Assassins.
Camila is also cool. Depends, you already have a bunch of Assassin so rather not.
Stheno? Do you want to build Sisters Charm team? If no, then no.
Fran is probably more deserved investment since AOE berserkers are actually rare to seen and very valuable for farming. She is glass cannon, but still quite huge damage for her NP anyway, better than Little Cat Tamamo
Beowulf? maybe not. No self-charge, laughable 2nd skill, and still quite glass.

Where is Lancers?.

If Fate/Stay taught us anything, lancers are the worst class. (Just kidding, I love you, Cú <3)

Yes you should enhance some of your 4 star servants. If Nerofest taught us anything, it's that it's good to have a wide array of leveled servants when taking on difficult content; particularly servants that fill a specific niche. Even Stheno has her uses.

If you are planning to roll for Merlin, definitely enhance Yuri archer. They are one of the few rare single target buster np archer and they hit hard with a

19.5% party attack buff for 3 turns on 5 turn CD
25.5% self attack buff for 3 turns on 5 turn CD with a no time limit guts skill attached

Just raise Siegfried, D'eon and some berserker ( jojo's meme choose Beowulf, farm power choose Frans ) also i hope you get lucky when rolling for some Lancer.
Your Supp, Tanks, and DPS is good already. Only lacking in AOE saber, AOE lancer, add more leveled berserker to your main team, also having multi Tank not gonna let you down.
Fionn is an Exception -_- DW please buff him, his kit is a mess.