Gradius 3 & 4 Confirmed Working
Just managed to get Gradius 3 & 4 working. There's a small caveat to my notes here though: as my original G3&4 has a physical crack on the disc, I used a backup (EA protection cracked) copy of the game to do the install.
Use the standard "CDRWin trick" as described with Contra: Shattered Soldier, meaning image the disc with 'replace sectors' instead of 'ignore', then install via HDL_Dump. You may need to obtain a crack and apply it to the disc, though I doubt it since it had pretty much the same protection as Contra: Shattered Soldier.
Now I just need to figure out how to get Gradius 5 working..