This produces a towering tree unless sheared into a formal hedge, or otherwise significantly contained. So, if you are going to use it to form a close hedge of manageable size, 4' apart is not too close.

And could even be a bit far, depending on what size and shape of screen is intended to be developed.

In Z8 hot dry Tx, my wife had 4 ft. Leylands planted 6' apart. They are at 15' plus where healthy with spread of ~12' after 5-6 yrs. I'd recommend planting them at 12' apart for better tree health. For faster screening, try 6' apart and then cut down every other tree in 3-4 yrs.
Yes to mulch. Locate trees at least 6' from concrete.

After seeing some pictures of these mature trees I am having second thoughts. I wanted a quick growing hedge not a humongous tree.

Would they be difficult to trim and keep at a manageable height? I want a screen not a forest. I purchased 8 of them for a 30' long space. I live in a residential neighborhood not a rural one.

Makes a nice, tight sheared surface and can be grown that way for some time in a comparatively small size.

If you keep up with the shearing.

If you don't want to have a formal hedge then HUMONGOUS is the name of the game here. Maybe plant something else instead.

Yep, humongous is the word! This Leyland Cypress is 36 metres tall, and with a trunk over a metre thick ;-)

Resin