1. They would get some, but with 4x 70s doing a bulk of the DPS you will get *very little* XP.
2. You will get a little more XP, but still little. You would roughly get 50% of the XP. It appears to scale linearly with how much damage you do to the mob.
3. If the mob is green to the 70s, then your solo toon will get full XP. Tag and kill will grant you full XP.
4. You will get the same XP as #3.

1 & 2 is based off my attempts to do the same thing. 3 & 4 I have confirmed on a few occasions (started getting full XP a bit into outlands). There is a page on the wiki that explains boosting with pets with full XP, but it only works when you have more than two toons you are boosting.

Best bet is to use your 70s to nuke quest mobs and just rip through your quests. If you are fortunate enough to be levelling a paladin, spec it prot and make a very large pull and heal with a lvl 70 toon. When your prot pally has done a large portion of damage then nuke the rest with your 70s. I levelled my prot paladin this way and visited all the typical AOE locations when mobs were all orange. As you may have guessed, instance runs are not worth it if you bring in 4 70s. Just do them for gear runs (if you need it). Level solo as much as you can tolerate with just a healer on tow.