3 or 4 weeks absence would seem to require a fish sitter, IMO.
Not an every day thing, but 3 or 4 times a week should do it. I agree that a lightly stocked tank would do better in this situation than an heavily stocked one. 4 weeks might require a water top up, but since it is just a top up, I would use the water right out of the tap if you do not have chloramines (a small addition of chlorinated water will be diluted, and the chlorine will out-gas before causing harm). this would keep it as simple as possible for your sitter (ie: refill tank to this mark). You can go one month without a water change if the tank is well maintained previously, just do a water change before you leave and when you get back.
I understand that there are many who value their privacy highly, and would never have peace of mind knowing someone else is wandering about their house while they are gone for a month. If this describes you, then the only thing you can do is fill the tank to it highest limit, use a battery operated feeder, and hope for the best. But I think that the peace of mind of knowing your tank is being looked after should count a lot too.