Excellent decision to replace AAA batteries with AGM batteries. A 4.5AH 6V battery will supply 4.5 A for 1 hour, or 2.25 A for 2 hours, etc., before recharge. One could build a simple charger, using the fact that for this case, the maximum charging current would be 0.45 A, or 450 mA. Note that VRLA/ sealed lead acid batteries are VERY rugged.
not quite. the 4.5ah rating is at the 20hr discharge rate. the ah rating will be lower at faster discharge rates. also hr needs a float charger that will provide a maximum of 450ma *and* limit the output voltage. it's not hard with an lm317, but for longest life and best performance (especially if you are likely to forget to remove the charger when finished) you should get a small 3 stage charger that is sized for this battery.
It is really best to current limit supplies to diodes. There are COTS chips for this. If you don't mind low efficiency, I think there is some hack of a shunt regulator to make it into a current limiting device.
The AAA batteries have enough internal resistance to drive (some) LEDs in parallel with the 'right' current. It's wasteful, though, because battery capacity must be derated for the high drain condition.
With 4.5V to play with, you could use a voltage reference, amplifier, and pass transistor to regulate the current. Or, you could use a series resistor (different resistor for each of the LED arrays, they match because they're thermally close). Or, you could use a two-transistor, three resistor current mirror (different mirror for each of the LED arrays). Or, you could go with a full switchmode regulated LED drive (overkill, IMHO).
If you want long battery life in emergency, consider a bright/dim switch, too.
You can buy simple switching power supply modules for LEDs that are 80% to 90% efficient. That'a bit better than throwing out the extra voltage (75%).
AGM? Ugh. They're heavy, fragile, and their efficiency is awful except at ultra low discharge rates. Given a choice between AGM and Alkaline for portable, I'd go with alkaline batteries. "D" cells are 10 AH for heavy loads and over 16 AH at lighter loads.
Lithium iron phosphate is usually used to directly replace AGM. NiMH is OK if you can charge indoors with no load.