So, whilst everything was unplugged, I thought I'd do a compression test (psi).
cyl - psi
1 - 130
2 - 120
3 - 130
4 - 120
At least they're consistent as opposed pairs, although I'm guessing it's better to have them matching as left and right pairs? Unless someone says these numbers are iffy, I plan to leave everything untouched before I build everything back together.