Croagunk was there, in the beginning Poison Jabbing Brock when he dreamed of a girl. Poor guy. I guess Croagunk really are strict, when his own master can´t even dreaming of a girl without being hit by Poison jab.Best part about that scene was how they played "A Meeting and a Parting". Brilliant song, for a brilliant scene.
Also, Croagunk never appeared in Brock's last episode!
Regarding if they thought about an E4 arc, no I don´t think so. They gave various members their own eps but I don´t think that they intended them to appear in an ongoing mini- arc. I think we got to see pretty much of them anyway so they didn´t need a separate arc.