Every job can be of use in Promyvion and the other missions. You just need to have a good balance. By far the most valuable jobs are Ninja and Ranjer. Promyvion is caped at lvl 30. BST is probably the only job that can't be of much use here, but if you have access to BST, you have another job at lvl 30 anyways use that.
Chapter 4 is going to end up being the "bare minimum" for CoP over the next few months. Chapter 4 gives you access to delivery system and Auction House in Tavnazia Stronghold (linked to Jueno AH), as well it unlocks more quests in Tavnazia. There seem to be some nice places to party at the 30 levels here, so being able to set up home in Tavnazia is very useful. You do get nomad moogle access as soon as you go there, without any missions. There are 5 total BCNM fights to get to chapter 4, including the 3 Promyvion ones I dont know if I would call these harder than Promyvion, mainly becasue being lvl 40, everyone has access to alot more skills/spells/etc, but they are still qutie tough, and word is they keep getting tougher. Challenge was definatly a key factor in programing. The last two are capped at lvl 40. There is 1 lvl 40 capped NM fight, but it is not in BC and can be done in alliance, and there are 2 soloable NM's (To LVL 65) in non capped areas. There is a LOT of traveling involved. It's kind of like Journey Abroad, part 2. Many cut scenes, which are very enjoyable as the NPC cast for this expansion is very entertaining.
I don't think these fight will be "hard" for long. As it's new, there aren't many walktrhoughs, people going in now really are the daring ones trying these things without a firm strategy in place. Especially now that nerfs have caused the entire battle strategies for some of them to change completly. With Zilart, it wasnt that hard as Zilart wasn't capped. But even the last few Zilart missions still dont have a real firm strategy in place. In a few months im sure Promyvion will be much easier once solid strategies are in place. And even promyvion isnt that hard, problem with Promyvion is lvl 30 people think they can do it, problem is most first time lvl 30 folks have never used or cant afford food and potions, whcih are needed.
EDIT : I said they sucked, I didnt say I was quitting. The Expansion is still great, the storyline and new areas are great.