Answer 1!!
Seriously, not a slip really. Arguably a little confusing to a foreign crew. However, I think you will find that the 4000ft part is a restriction, before which you can not be taken off the SID (noise). As the transition Altitude is 6000ft, it makes sense to me.
Seems like the controller was, as is often the case, just trying to keep you climbing and give you a direct routing.
Stating the freq/aiport is not a sin, and neither will you be flamed for it. You are more likely to have an exact answer if you gice us the info!
Maybe it is something that, from a Human Factors pint of view should be discouraged if proving confusing. However, it is heard a lot and I don't think causes level busts or losses of separation.