Have just switched from an Apple Airport Extreme running my Wifi through a HH3 with no issues.
Decided to swap to a HH4 as it meant only 1 device instead of 2.
Been fairly pleased until using FaceTime, the video is really poor out to my daughter.
Have swapped the Airport Extreme back into the system and bingo all back to normal.
Are there some Ports the HH4 is blocking that the Apple protocol needs.
Try logging into the HH4 and see if you Apple Mac computer has a device name with spaces or : characters. this seems to cause a lot of network related issues. If you do find such a name listed you need to change the name in your Mac.
Go to System Settings > Sharing Click edit in the Computer name box and enter a name or just change the name slightly (but no spaces). Then restart your mac and log backinto the Home Hub to confirm the name is now seen without od chaacters or spaces.
This problem seems to occur most with Macs that have been upgraded from previous OS X versions, as it seems to use a computer name with MAC addess added.