So, yes and no as far as the simplicity. LED's are often viewed as fixed resistance, classical devices with completely static power draw, or at least only proportional to the square of the input voltage. The reality is that since they are in fact diodes, the amount of power they used is governed by a nasty, sorta nonlinear relationship to the input voltage. (...damn semiconductor physics)

Where this applies to your application is the following:
You've quoted your LED's as being two volts each; because of the reasoning above, LED's like most other semiconductor devices are really rated by current. As a user of LED's your goal in life is to make sure the current flowing through the LED's is what they should be, which when used individually should be two volts. However, because of the nature of variable resistance devices, placing them in series and hoping for a fixed-resistance aggregate effect is not always guaranteed. Variances in the LED's themselves, both from manufacturing inconsistencies and the usage of different but equally labeled parts can account for several problems; problems symptomatic of unequal luminous intensity and possibly the burning out of the part itself.

Without a current meter, I present you with the following solutions:

1) The practical route: All the dangers shouldn't be a problem, and if they're really rated for 2v each, an additional .2v shouldn't be a big deal, but who knows. If you burn down your house and soil your pants, I warned you.

2) The practical safe route: Assume the principles of option 1, stack them in series, and use a variety of power supplies to ease it up to 9v. Start low (not too low, because there is a minimum voltage (Vt) that you have to exceed for electrons to flow through the semiconductor), and make sure that each LED is about equally bright and no brighter than they would be when used alone. The scientific version of this approach involves using a multimeter, and that when driven in a series chain, the voltage across any one part is no greater (or not much greater) than 2 volts each (say, + or - .3 volts)

3) The safe route: Give each his own resistor, so, four LED's in series with his own resistor; with each LED-resistor pair in parallel with each other against the 9 volt source.

...my pick, #2. You'll learn more, and undoubtedly have better battery life (less wasted in resistors).