I was thinking about over seeding in the back because lawn is thin there, but then can't use herbicide for a couple of months, right?
That's correct. But if you kill the weeds....the grass will be healthier and may fill in some small areas.
You can then seed over any bare areas as needed after 4 weeks.
Yep, you can overseed in the spring, but shouldn't use preemergent (I'm assuming cool season grasses here). You can use post emergent (weed and feed) after you've mowed the new seedlings at least 3 times at their preferred height. That means it you're planting tall fescue, you'll mow around 2.5-3 inches and a couple more weeks of mowing at that height before using a post emergent.
Keep in mind that seeding cool season grasses (fescue, rye, bluegrass) in the spring can help fill in thin spots now, but most of those seedlings will not survive the summer. They just won't be mature enough to develop an energy reserve and will likely die out. Fall seeding is ideal, as it allows more time for them to mature before summer stress.
However, seeding in the spring can help keep some weeds developing, but think of it as a temporary fix and plan to seed in the fall.