Ahhh... Rank 4 and 5 were both extremely easy, although fairly long.
Both of them, my LS at the time took ~9 members out to do each quest. Delkfutt's was easy--we crushed everything in our way, having 9 members between level 35 and 50. For Rank 5 we had similar members, the same amount but a little higher levels (not much higher, since we did Rank 4 one weekend, and Rank 5 the following).
Rank 5, I was extremely helpful. Out of the 9 people, I was the only WHM (I think, might have been one other), so I was necessary for the two Machines (Silence and Curse). I got silenced (of course) and brought two Echo Drops. Used the Echo Drops and Silena/Cursna anyone that needed it. The only bad part was waiting on the NM's to drop the items. With 10 minutes or so to spare between each spawn, we had tons of time to sit around. Even messing around with how well each of us could do against some of the Quadavs solo. LOL (not so well here...)
Ahhhh... Let's see...
Only extremely difficult things I've encountered....well, let's see....
I had a run-in with Bloodtear Baldurf (?) in La Theine as a level 18 monk. LMAO
Squish!
Well, it was stupid. And I didn't know any better. Knew it was an NM. Just not how high level it was. Told one of the other members of my LS that was higher than me (probably level 30?) ... he was smashed in two hits, instead of my one. lol -- Lessons Learned....
so .. Yeah, that sucked.
BCNM20 sucked too, but we had a bad party for it. MNK, WHM, RDM... and BST, WHM/bst, RDM... Didn't work so well.

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46whm / 23blm - 33brd / whm - Rank 5, San d'Oria - 61.3 Woodworking - 12.0 Alchemy
20bst, 18mnk, 11rdm, All Others Unlocked (Lv 10 or Less)
http://ffxi.allakhazam.com/db/poster.html?user=322919 (WHM, BRD, RDM, and BST listed)