I have access to both, but the CNC is old and you have to program it on the machine itself. I would also have to learn how (I'm a software engineer, shouldn't be too hard).

If only doing 3 or 4, which is the most time effective?

An 80% ? Vertical end mill will be more than sufficient.

I just wondered if the time investment on programming the CNC would pay off if only doing 4. Or if the mill would be less time.

Then again, I might want more in the future, so it might be worth it.

If you're proficient at programming and consider this aspect "fun" then sure, CNC.
An 80% lower can easily be finished with the end mill.

If it's for fun and time is of no concern, then choose the one that will give you the most satisfaction.

An "end mill" is a cutting tool, not a machine. It makes you sound like a real dumbass to those who know better.

This makes you sound like an arrogant prick, even to those that know better.

For the OP; you'll probably spend more time learning G-code than it would take to setup and turn the wheels on a manual knee mill. My experience machining raw forgings was an enjoyable learning experience, not a fine tuned venture in maximizing man-hours efficiently. If you want to learn G-code, taking a stab at 4 lowers is a good opportunity. If you want to machine 4 lowers in the shortest time, do it manually.

1) Being a software engineer has absolutely no relevance to programming a tool path
2) Have you created an exact 3-D model of the lower or done a shitload of math to get the needed numbers to enter a tool path into the controller
3) If you want to do 3 or 4 lowers, did you purchase 6 or 8 so you'll have enough left after you throw a few away because generating a proper tool path by any means takes some experimentation
4) If you bought 80% lowers to be "time effective" somebody lied to you

Go with the manual mill and do some practicing first unless you don't mind throwing a couple forgings out
If you are dead set on cnc message me and I may be able to help you out.  At least with 3D modeling and possibly some of the tool path

Being a electronic/computer engineer... I would check into that CNC mill a little more.
All the manual entry systems I have seen, had a serial port you could access..
If so you can create the code for the specific system you have using a CAM package.
Then transfer the CNC code directly to, or through a laptop...
For doing multiple lowers in one session, it' the only way to go....

Here is a photo of my little CNC mill doing it's job on one of my lowers.....