The counter point would be what does 4 AP add? Diana's Q gains 2.8 Damage (2.4 on Passive and W), Lux QE gains 5.2 Damage. 3 AD adds ~2 Damage per hit and is mana free and not restricted by cooldowns (or Mana).
Anyway, as for last hitting, Melee Minions do 12 Damage @ 1.25 APS, Caster Minions do 25 Damage @ 0.67 APS (15, 17 DPS); health wise, it's 445 and 280 respectively*. Most AP Mids have around 50 AD at Level 1 and gain approximately 3 per Level.
That said, it's hard to give a total DPS figure since minions don't focus fire and they're rather unpredictable. But, then again, that unpredictable nature is sort of the point. The different attack speeds (plus just different starting times for attacking) make it hard to gauge exactly when their health is at a certain level unless you check their health by clicking on them. Every point of AD is a little extra leeway in eyeballing health.
*Their Damage, Health, and Armor scale up as time goes by but you get the idea.
Most people don't really run much besides AP Quints, if that (Movement is honestly a more attractive option most of the time). Generally speaking, if someone has more MR than you in lane, they're running non-MR blues which puts them at a greater disadvantage.