Instead of replacing the 4POS you may just get a Delta Stock addon... Which will give you a better fit... without having to take off the telescoping stock.. If you want sub MOA then get a 20" Barrel with Floating Handguards... YOu can do this two ways.. Go to Bushmaster and look at their complete uppers or you can by a barrel and get a floating handguard such as knights or something... HOwever, if it was me I would just call Bushmaster and get a VMATCH or Varmiter Upper. In a flattop or A3 configuration. The A3 config will give you a detachable Carry handle with 1/2 MOA peeps, and 600 yards.... If you go with a detachable carry handle there is an article on 50/200 BEZO that is very good and is alot better then the BEZO that most go with, it was written by a Marine and tells how to set up A2 sights to get this...
If you want it for plinking then I would get the 20" barrel...
Ben