I have been reading about greater dragons and stats some and have become confused. Am I suppose to be looking for a pre tame 5 slot or does he have a better stat cap if i tame it at 4 slots and then train to 5?
If you can get a 4 slot, train that one. At 5 slots all you can do is train its skills up to the max that they are when tamed. A 4 slot is able to go through the new training process and become a 5 slot. You will have one round to spruce it up - adjust resist, change its magic, give it some regens, give it a power scroll or two, etc. You won't be able to max everything out with just one round of training; so, I would give it a try on test center before doing it permanently.
Neither is useful since they are not mounts. They do 100% physical damage. A Lesser Hiryu will make a better pet and you can mount and does 100% physical damage plus they have Grasping claw. There are better magery pets than a Dragon. Magery does not make a great pet for someone that is new to training.
A four slot would be good to use to help you learn the training system since you can release them when you find better options.
the 5 slots aren't to bad if you are looking specifically for a brick wall of hitpoints to throw at mobs, but resists can't be changed.
i seem to remember 4 slots having really low hp(600ish) compared to capped 5 slots (1000)