I have their m15 model that is m4 config. it is nice for the money and has an ok 2 stage trigger, which is nice to not have to upgrade. several years ago guntests did an ar shoot out and said that armalites were a best buy quality for the buck.
don't know which trigger the a4b has...
I also had a ar-10t that was awesome, but it was not something I needed (26" heavy bbl) so I sold it.
Ruger makes a nice ar for the money. look at their MPR ar. it should bee 700-750 and has a free floated bbl (mlok forend), and ok upgraded trigger.
Ruger looks like you won't be tempted to upgrade stuff later