I was an Installer I until June (I am out of construction now, and I let the license expire.)
The rules can vary greatly from county to county. There are still counties where the 10 acre rule applies, and most counties will let you you install your own system without being licensed, though you have to ALL the work yourself except any design, soil evaul, etc.
$4500 sounds in the ball park, depending on your soil. I never did a system cheaper than $3000, but quite a few that were higher than your quoted price. If it is a standard system, then an Installer I can build it to spec, you do not have to have an engineer design it. (Though your county may have amended this rule, as is their wont)
Google TCEQ (Texas commission Environmental Quality.) On their site they have all of the rules and such on septic systems. Even if you do not build it yourself, there is good info to know when dealing with your contractor.