Hey, the 4D people i went with promised to try and not show you the sex if you didn't want to know but they pointed out that they couldn't guarantee you wouldn't see something - as obv it depends on babies position etc.
If they saw then they wouldn't need to tell you, so it would depend on how hard you looked really! I'm sure they can concentrate on the face for the 4d parts.
I'd say Go for it! xxx