First off, welcome to the Army youngster.
Glad to have you with us.
Next.........
Before you buy an M4 clone, compare prices at different places first.
A few examples......
Most posts sell M4 types at the PX, which means you wont be taxed to death during purchase.
A 5 day temp on post registration and your set until you can get the permanent one thru your COC.
Others include various gun stores that give Military discounts. This simply depends on which post you will be stationed at.
Last....... Stick with iron sights for practice. Some units allow optics for BRM and some insist on iron sights only. If you are using iron sights, then you will be ready for either later on. ( The better you shoot and better you do during your APFT will give you more promotion points ) Stick with the exact type of rear sight that your issued M4's use.
Also, stick with buying 30rd aluminum GI mags, as the Pmags are not authorized for use through Army weapons. (ie..... You get used to the mags , it will go towards more familiarity with your issued weapon and magazines.
Edit: You will still be required to shoot BRM with an M4 regardless of if you are assigned an M249 or M240.
Hope this helps some.
Good luck and keep your nose clean.