It sounds like the OP was asking which plane would be the easiest target to bring down once you have a gun solution. Of the aircraft mentioned, the spit usually will go down before the others. This has little to do with size but more with durability. A couple rounds in the wing root will snap it off peretty easy compaired to the others. Durability would probably go in this order from least durable to most...spit, 51, 38, 47.
If you're trying to pepper one from a dead 6 from long distance going from easiest to hit to hardest...47, spit, 38, 51.
The 38, while wide from tip to tip, is like a razor from a dead 6.
Top down while trying to score hits with a full profile easiest to hardest to hit...38, 47, spit, 51.
Side view hits easiest to hardest to score...47, 38. The spit and 51 are about equal.
Most to least durable..47, 38, 51, spit.
When entering an engagment with all 4. It really depends on my E state compaired to theirs. If I'm holding a large reserve of E when I come across a group like that I'll dive in and kill whoever isn't paying attention first. Then I'll work the other planes based on my priorities which usually focuses on the most dangerous pilot. Plane selection is usually a good indication of pilot skill. So I'll go after the 38 first because he's the most likely to have the most skill and most likely to pick me off if I get tied up. The other three are pretty much a toss up but I'd probably go in this order after the 38 is eliminated...
spit (cuz 20s are most likely to break my plane if I get hit)
Jug (because he won't be able to escape my jug)
51 (because his pathetic attempts to pick me have failed and now he has no bait and only two options...die or run)