The oil seems to be on the ends exposed to the outside only - i.e. not the ends inside the engine.
Possibly they're just the two nearest to the oil filler cap? (so when a bit has spilt in the past (during topup/oil change) its hit those plugs and not so much the others??)
If its the two at the tother end then lol
It's no suprise some oil has spilt in the past if the x-brace has been stubborn to come off
I wouldn't be worried about that. Now you have four matching plugs I wouldn't worry. Cant see where the oil has come from, but it's a tiny amount in the grander scheme of things. pop the new ones out in a couple of hundred miles or so and I suspect they will be spot on