How do I get 4-5 stars?

[SR]

I seem to keep getting low scores but I keep kills at minimal and usually just knock em out without killing, any tips to get better score?

[Kunovega]

"keep kills at minimal" ~ as in, you kill more than just the targets?

It should be kind of obvious that when you look at the score:

1. All objectives complete
2. NEVER spotted (doing anything illegal, tresspasing, carrying weapons in the wrong disguise, etc)
3. NO noticed kills (that means no one sees the target die, even if they didnt see you commit the crime)
4. NO bodies found
5. NO recordings (either never be seen on camera or destroy the evidence before leaving)
6. NEVER KILL ANYTHING THAT IS NOT A TARGET (notice the giant non target kill penalty?)

5 stars is Silent Assassin
https://steamcommunity.com/sharedfiles/filedetails/?id=1492033279

Or you can screw around and get none:
https://steamcommunity.com/sharedfiles/filedetails/?id=1460854579

Any mission can be done silent assassin as long as you only kill targets:
https://steamcommunity.com/sharedfiles/filedetails/?id=1410974603

Wait until you start trying to do challenges, Silent Assassin Suit Only requires all of that plus not changing your suit:
https://steamcommunity.com/sharedfiles/filedetails/?id=1268890624

[NecroMaster]

As Dianna said never draw attention to yourself.

The Mark of the true assassin is to be unseen and untouched.

[Mr_Faorry]

Pretty much the bonuses you see on the score screen tell you how.

The big one is non target kills, just avoid doing this, knock them out with a hammer if they're in your way. -5k score per kill.
Going unnoticed will also add to your score, so not being seen doing anything suspicious or illegal. +20k score.
No bodies found also adds to your score, bodies that were killed with an accident kill are allowed to be found though. (poison, falling, having something fall on them, non bomb explosions, drowning, fire and electrocution) +20k score.
Being spotted by a camera and not deleteing the evidence will give you a big score penalty. It's not too hard to delete the evidence since you can just shoot it with a silenced gun from around a corner. -20k score if you fail this.
Then of course finishing the mission faster gets you a higher bonus from that too, so finishing a level in 5 minutes will get you a higher score than finishing it in 30. This is easily the least important since it doesn't affect the score too much, just don't spend too much time in a mission.

Not sure if doing it silent assassin adds a score bonus since it appears on the results page with all the other score modifiers but it doesn't have any numbers with it, but doing all the above gets you silent assassin so it doesn't really matter.