I have several possible ideas, although I do not know which might be "best" in your situation. So I'll just list them all, and maybe you can use one or modify one to work.
First, you can make it safe enough to be away for that length of time.
--if the heat lamp cannot be knocked down by the chicks
(secure it, and be sure they are not yet flying)
--if the heat lamp cannot be knocked down by something else
(shut the room door to keep out dogs and cats)
--if the heat lamp is not heating any combustible material hot enough to catch fire
(look where it is shining, use a thermometer-- bedding at chick level at 100 degrees farenheit will not catch fire, but the cardboard edge of a box really close to the bulb might.)
In general, if it has been running for 12 hours without catching fire AND if nothing changes while you are away, I think it will be fine.
Second, for up to 12 hours or so, in a centrally heated house, they might be fine if you pack them in a box the way they would be packed for shipping through the mail. But I do not know for sure--chicks that get mailed are usually packed in special boxes, and it usually takes about 15 chicks or more to keep themselves warm. Because you have only 4, it might not work right.
Third, you could look into the "momma heating pad" idea that is in some articles on this site.
It is often recommended as being safer (less likely to cause fire) than a heat lamp. It does use electricity, but it sounded to me like you were concerned about fire safety more than actual electricity.
Fourth, for a short time, it might work to put a large jar of hot water or a hot water bottle in one corner of a large brooder, and let the chicks self-regulate their own temperature. But I don't know if it would hold heat well enough for 3+ hours. Maybe if you started with a 5-gallon bucket of very hot water, and the chicks were initially on the far side of the brooder (almost too hot), it might still have enough heat by the time you came back (they'd be pressed against it then). But I'm really not sure. It's the kind of thing that you would be better off testing in advance to see how it works.
Fifth, there exist heat plates for chick brooding that are controlled by a thermostat. Still electric, but supposed to prevent fires from overheating. It might not be possible to get one in time for your event. They are also quite a bit more expensive to purchase (as compared with using a heat lamp you already posses.)
Personally, I would just make sure the heat lamp is as safe as you can get it, and has not been changed in the 12 hours or so before you leave. Combined with penning any dogs, cats, etc away from the area so they cannot bump the heat lamp.
