Yes, 4g is for data only. When you get a call, it uses 2g/3g, thus you can talk and surf at the same time. Don't know about VOIP, but I would assume it would use 4g which would eliminate surfing while talking (on skype for instance). But I've been surfing many times and taken a call and just kept on surfin'.
Take that, AT&T!
And yes, you can turn off 4g, and it won't affect calls in the least. Though if you are surfing, and take a call with 4g turned off, you will not have a data connection.
The ONLY reason I would turn off 4g however is if I was in a border area and the constant searching for 4g was killing battery. If you are in a strong 4g area, no reason to turn it off, as it would only drain battery when you are using 4g (since it wouldn't need to be searching for a 4g signal). If in an area with no 4g, I guess I'd turn it off as well since it wouldn't be doing anything anyway and would probably keep searching as well.