I’ve machines lots of 4140/4150…and will be working with some in a week or two. My recommendation is to machine it in the annealed (soft) state and then heat treat it to the desired hardness. I know not everyone has this capability (ability). If you need to work within the hard condition…umm, well good luck. Possible? Sure, but it will be a lot slower and tougher on the machine/cutters
Coolant (Amazon)? See the photo. I mix it 10:1.
My material (unfortunately) is already hard, so I will be annealing it, machining it, then rehardening it (tempering it too of course), but that’s the way I roll.