I had 4D scans done with both Tehya and Abbey at SUFW Norwest and found them great. I think the difference is though I was in for a sticky beak scan, not a morphology scan which is what the 20 week scan is.

If you are just after a good look at bub in 4d then I would suggest waiting. The bigger they are, between 28 and 32 weeks the better the pic you will get.

SIL also went to the same place and they were great with her.
Oh with Tehya I forgot my video, we were nearly finished the scan when I remebered. She finished up, told us to go get the tape while she did the next couple and to come back and she'd get some footage for us. I really cannot fault their service there.