Now (right after flowering) is the time to prune and perhaps a week or so more. If they really are 4ft sticks, you should be able to prune 1-2 ft off the top. They may not leaf out much this year, but if you want to bush them out, that's what you should do. It is a common practice not to cut more than a fourth of the foliage area off at one time, or you risk the plant going into "shock". Lilacs are just picky this way, but I think you will find it does not apply to such a young specimen. A lilac in shock still grows fine, but just doesn't flower for 2-4 years. Some lilacs, even many of the selected named ones, will produce suckers from the ground. Some don't. (I am assuming you are talking about the regular type of lilacs, not Chinese, Japanese, dwarves.)

The "inside branches" are meant for older bushes, and actually is a technique meant to produce a tree like shrub. You are right, it does not apply to you.

It is normal for lilacs to stop growing in late June/early July. That's just the way they are.