Most equipment manufactures have added some degree of adjustability to their drivers for 2013.
The new Cobra Amp Cell, Cleveland Classic XL, Ping G25, Nike VR S Covert, Callaway Razr Fit Xtreme and Titleist's 913 drivers all feature adjust ability as well as the market innovator in the area of adjust ability Taylormade with their new R1.
Will changing your loft by 3 or 4 degrees make much difference?
Yes, is the simple answer, up to 70% of golfers are currently using a driver with the wrong loft. In a recent study conducted by Cobra they found that increasing you launch angle by 1.5 degrees will in fact add up to 18 yards to your drives. Most golfers need more loft not less.
What am I adjusting on the driver exactly?
The clubs loft, lie and face angle can all be adjusted via the hosel. Generally a loft change will change the lie, making the club face more open with higher loft and more closed with a lower loft angle.
Adjustability with measurability, ie knowing what you are adjusting your club to is important. Adjusting/tinkering and not knowing what setting your driver is at will not help you understand what works best for you.
By being able to know what you are setting your driver to it will help you to get a better understanding of what works best for you in terms of loft, lie and face angle.
Don't be afraid to try out different settings, as your swing improves you might need to make a slight adjustment to your driver (rather than buying a new club).