What are the pros and cons of using 5 cells instead of 4? Looks like it might fit and it is under the 7 volt maximum. I am so used to 9.6 volt transmitters that 4.8 volts scares me a little.

I do not have access to the schematics for the transmitter but; from an engineering standpoint, this would not be a good idea. Power output is limited by law (at least in the USA) and this is usually done by the design using the voltage stated by the designer. Any increase in voltage will not increase the power as the voltage to the final is limited by the circuit design. All the excess voltage will do is dissipate unwanted heat and strain on the components used to regulate the power output. In short, the added cell will have adverse effect on reliability and life span. I'm sure if this is wrong, you will get a response from the manufacturer rep or other knowledgeable source.

The transmitter comes with a 4-cell battery pack. Don't use one that's different. A 5-cell pack will deliver too much voltage. Using one may damage your transmitter and will certainly void any warranty.

Thanks for the excellent answers. The 5 cell looked like it might fit, but it wouldn't. So I'm a happy 4.8 flyer.