Welcome to CPF guts, and why dontcha stay awhile!
Wow, if the GL4 is your first light, that's an a$$kicker! I am unqualified to say any more, never having seen a GL4, so I will go on.
The GL4 runs on 4x123 or rechargeables, so if you don't have one or the other, that's an upgrade. If you are running it off of lithiums, it makes sense to get the rechargeables, since it will cost you about five bucks to load up. On the other hand, while the rechargeables remove the cost worry, you should hang on to a handful of 123s so in an emergency and the rechargeables are kaput, you can pop in the lithiums and are good to go.
Next, I dunno what kind of tailcap you got, but they come in plain clicky and different LED varieties, like red, green, white. I prefer the white ones, for the best color rendition, but you may have other needs, like red to preserve night vision.
I'm sure you have other flashlights, so I don't need to tell you that most flashlights are less bright than the GL4. Keep in mind that the rechargeables give less lumens than the lithiums, but longer runtime.
What comes to mind immediately, if you are trying to beat the GL4 are the three and four cell M series from Surefire, which can put out 225 lumens or 350 lumens, depending on battery and lamp configuration (compared to 195 from the Pila and 4x123). And with the Surefires, you are talking considerably more moolah, too.
welcome again,
daloosh