A receiver can go into many different planes.
Do you want to end up with a receiver that is only useable in a 4-channel plane? Or one that might be able to go into something more sophisticated/advanced at some point in the future?
Do you have any plans of getting a new transmitter in the future?... again just a thought.. spending a few more bucks today might save a few in the future.. A 4-ch. receiver limits any future projects.
Multi-channel receivers have several options. They allow for dual aileron servo mixing... which would include flaps, ailevators, elevons, spoilers, elev/flap mixing, and maybe a few more I'm forgetting.. Not all of the functions can be used at the same time on a DX6i... but every one of these can be used on any given model one function(maybe 2..) at a time... Its the receiver that would make that possible.
I don't think I'd even look at a 4ch.Rx with all the options that are available with a 6+ch Rx... and the mixing options are very important with giant-scale.. I have a 4-ch. aerobat.. using 9channel receiver... everything runs direct and mixes in the transmitter.. thats another story...
I don't know what the cost factor is going from 4ch rx to a 6ch.. but I do believe the value is priceless in this hobby. In any case, I'm sure you'll do fine and get just what you need... This is just my 2cents... I wish you well.
