hmmm 4-6 weeks is a long time!
what tanks and fish do you have... some will be easier than others.
a few general pointers regarding holidays are a sfollows, i'll maybe tailor this a little to your set up when i know what you've got.
firstly most community fish will be fine up to 2 weeks without feeding.
don't use holiday food blocks, they will pollute your tank and you won't be there to sort it out.
by far the best thing to do is get someone in to feed your fish, if you don't know anyone try looking through the phone book for people offering they're services as dog sitters or things like that, could be worth a try. sort out portions of food for each tank, wrap them up in foil so all they have to do is put one packet of food in every other day, then they won't over/under feed. for a longer holiday you could also ask them to add tetra easy balance to the water weekly, this is supposed to reduce the need for water changes, i wouldn't advise using it all the time but as a 1 off for a few weeks it should help.
if you can't get anyone in to feed them get an automatic feeder, this drops a set amount of food into the tank at set intervals, however they are a bit unreliable, test it by running it on the tank for a couple of weeks while your at home and can see that it's working.
resist the temptation to give them a big feed before you go, this will result in more mess and waste and it means the water quality will deteriorate quicker.
give them the last feed the night before you go, an hr or so after you should do a decent size water change and gravel vac.
hope that all helps a bit!
