I guess the reason is the hardware you have.
I have experience with the 2000s, that after you apply an add-on key, you make it a 2200s.
Apparently, the 4000s look to be the same, as the hardware is the same as 4200v.
A major difference, in both 2000 or 4000 series, is the compression, either been via software or hardware.
I guess that is the reason for the limited license, as you only have software compression, and that is seen as a limitation for AAM.
Anyway, best way to confirm that is talk with F5 sales team.
They can confirm, and also provide you the price of the add-on if is available.
Here is the hardware datasheet:
http://www.f5.com/pdf/products/big-ip-platforms-datasheet.pdf
F5 Sales team contact:
https://f5.com/products/how-to-buy