Best trigger upgrade is putting the 3.5# connector, swap all the springs and do the 25c trigger job. Very effective and cheap upgrade. Did on all my glocks and the trigger feel are great, light and crisp.
If you are mechanically inclined go online and watch the YouTube "25 cent trigger job" as suggested. You can buy the Glock parts online from Brownells or Midway. You will need the 3.5 pound connector, 6 pound trigger spring and safety spring. Changing out the striker spring will lighten the pull even more, but may cause light striking on the primers. All you need is a Glock tool for dis-assembly. You will need white cake polishing compound and a roto-tool with small buffer. If you are not mechanically inclined I would find a Glock armorer at a gun show and let him do the trigger job.
If this is for carry. The best upgrade is training. You will find that the "." connector that comes in the Gen 4 is just fine. As far as those skimmer triggers or anything that takes up the pre-travel, you need to be very careful as there are reports of those things neutralizing the drop safety.
The best Glock trigger upgrade is a case of ammo down range. Break it in and train your finger. This whole topic reminds me of the new AR owner who wants to know what upgrades he can do. Kind of like chroming out our brand new car, buying new wheels and tires, installing a new "bangin'" sound system, a turbo charger, auxillary oil and transmission cooler, Recaro bucket seats and new steering wheel before you even drive it AND, never ever race it.