Well, I have created a pattern, and I want it to have four layers. My problem is that when I try to save it, i get an error message saying "too many channels (5)"
How can I solve this?

Five channels is really too many. How do you create the alpha channel?

I create it in the channels tab of photoshop, clicking the create new channel icon.
I have in total the RGB changel, the red, the green and the blue channel... and the alpha.

I have no idea about Photoshop, but clicking the "new channel" icon in GIMP brings similar problems. In GIMP, you either right-click a layer and select "add alpha channel" or create a transparent image (which automatically has alpha)

I think that creating a new transparent image in Photoshop should create an alpha channel automatically.

Make sure your image is flattened (in the layers palette menu). It needs to have just one layer, and it needs to show up in the layers tab as "background" not "layer 0" or "layer 1". Any non-background layer has transparency info and that's where the fifth channel comes from. This true even the layer doesn't have any transparent parts and there are no other layers.

Ok, BabaYaga, incredibly, that was the problem, this was driving me crazy, because I did not have other layer. Thank you.