Are you referring to the 13" or 15" rMBP? Either will last 4 years (the 15" a bit better because of the quad-core CPU and discrete GPU) although that will also require fighting your urge to have the latest and greatest since there will certainly be a lot of improvements over that time (I predict "Retina-quality" displays will become mainstream within 2 years throughout the Windows and Mac world).
That said, as others have pointed out, the MacBook Air and cMBP are also very good in their current forms. It's nice not to incur more debt than necessary, particularly between ages 18-22. A nice rule of thumb is to buy something only if you can pay the credit card bill in full the next month (and then do so). I like paying with credit cards for the warranty benefits, but I pay in full every month. I'd avoid the Barclaycard that they push in the Apple Store. The benefits are virtually nonexistent, there are no rewards (except the very first time you use it), and it's too easy to be lured into the trap of the interest rate deferral.
Since you are a student, I'd suggest looking the refurbished models if you do decide to go with the rMBP - or any Mac for that matter. The discount is bigger than the education discount and they are like new except the box. Don't worry too much about getting CPU upgrades (although the $85 upgrade from the 2.3GHz to the 2.6GHz in the 15" isn't bad if one shows up in the refurb store).
I'd recommend this as well. It's called a personal articles policy and can usually be purchased from the company who provides you or your parent's renter's/homeowner's insurance policy. Mine is $60/year and covers everything from theft, loss, accidental damage, fire/water damage and mechanical failure. I've had the policy for a few years now. It came in handy when I lost my MacBook Air last month while traveling. I wound up replacing it with the 13" rMBP.