Sold/Expired Problems with my batteries

I bought a light for my bicycle a couple days ago that took a lead acid battery that is in the shape of a water bottle the whole system is huge like 1200 grams. I went to radio shack and bought a battery pack that had the wires hanging out of it and the correct plug and plugged 4 aa batteries into the light. The light is not nearly as bright as what it was with teh rechargable water bottle style battery that came with it. Are the batteries not high enough in amps or volts or what? The stock battery is 6 volts so I figured 4 AA's would be good enough...

SLA actually are a little more than 2.1v/cell with a 6v battery being approx 6.3v and when charged they can end up closer to 7v. The 4AAs probably are 1.5v but will sag under the heavy lead of the lamp most likely causing a voltage drop to perhaps closer to 5v. If you want to see what is goin on put a voltmeter on the batteries and turn the light on and watch the voltage sag. One thing you could do if the voltage sags a lot is add a 5th battery to the mix.