4 Star Buster Servants to roll

In the future or what? Because there's Saber Alter right now if you want a good buster SR.

Are you asking about current or future Servants?

For currently available Servants and Servants that have been released already, Herc, Ibaraki, Rama, Elizabeth Brave, VladIII(Extra), Kiyohime Lancer and Martha Ruler are pretty good, especially Herc. Martha and Kiyo Lancer are limited Summer Servants who get one last rate-up this year during their summer event. Eli Brave is a welfare who gets a rerun this year as well. In the end though it also depends on what you're after.

The ones I mentioned have ST NPs so are good for dealing with bosses and such but not so much for farming. AoE Servants would include Saber Arturia Alter, Lancer Arturia Alter, Nobunaga (welfare with a rerun in a few weeks) and Gawain.

Can't remember if I'm missing anyone notable.

Passionlip ccc event aoe buster

Raiko Lancer in this year's summer banner

NA already have lots of good Buster SR servants and for reference here's some upcoming Buster servants. Raikou Lancer, Suzuka Gozen and Penthesilea or Nobu Zerker or etc are pretty good if you're lacking solid servants from that department.
For Buster welfare, they're exceptionally good and unit such as Nobunaga Archer or Cha-cha Zerker or Saber Elizabeth will be available soon.

Premiere one : Hercules, Salter, Lalter, Raikou Lancer, Ibaraki, Carmilla. If you got one of them then gameplay wise you don't need another one from the same job. Hercules are equal to Chu Alter and better than most 5* Zerker, Ibaraki only below Hercules, Carmilla only below King Hassan and Jack, Salter only below Altera, for now Lalter don't have any competent rival on the same department ( AOE Lancer ), Raikou Lancer pretty much just like Brynhildr and Tamamo Shark. Mentioned servants here mostly hit like a freaking truck and also their performance is similar or almost just the same as 5* servants, i would say they're still good unit to roll for long term Investment.