5 things that should help:
1. Fly at least 50' away from each other, essential.
2. Since the P4 uses the same Lightbridge technology as P3P (both using GL300C controller), each of you should select a custom channel and set transmission quality to 4Mb/sec.
3. Set one RC on the lowest custom channel that has the least traffic, and set the other RC on the highest channel with the least traffic.
4. Try using the 32ch hack, which will help you get your frequencies farther apart in the 2.4Ghz range.
5. Use a
$10 windsurfer on each RC to help shield against interference from the other controller. It will increase range also.