Planted tank will be most helpful, as it will break up territories, line of vision and several things. Plus the love to ambush their prey...lol...so it would be most appreciated. TONS of driftwood, can't over emphasize this. They are cavity spawners, so they will probably spawn under pieces of driftwood and the like. Plus the driftwood will lower the pH and acidity of the water, both very important things with spawnin dwarfs or large pikes for that matter. It can be done in harder, higher pH water but its more difficult. A way you can condition them for breedin would to be to feed them live worms, like blackworms, bloodworm, and whiteworms. These seem to prime them up for breeding and one of the main things that gets them going.
I have never kept dwarfs myself, so I don't know their aggression really, but I am assumin they are just like other pikes, if not worse. So keepin two pairs in the same tank might be a bit of a struggle. Once they pair up and start spawnin and stuff your gonna have problems...lol. You might be able to keep 1 male and 2 females. The extra female will be kinda like a dither to help strengthen the pairs bond. Just watch it though cuz they could and would kill the extra one. You might be able to get away with it in a 75gal though. Give it a try, enough hiding places and such might help it out a lot. I would get at least 4...that way you have a chance of gettin a pair better. They are gonna be pretty aggressive with each other until they pair up...lol...so you may be bitin your nails for a bit. Just make sure you have some PVC in there so they can hide and get away. Rapps has some NICE regani in right now, so your set!