My car is 4 years old and have never been rust checked.
I was told not to apply rust check as it would only seal in any rust and cause more damage.
Is this right ?

Most, and I say most late model vehicles, including your's, probably already has the protection that you seek. You best resource would be to contact the manufacturer and ask about this.
Like I said, most late model vehicles already have forms of protection that don't require additional protections, to the point of offering substantial outer body rust-thru warranties that exceed the powertrain warranty.
I encourage you to check with your local dealer or the manufacture's website. You may even want to call their customer service.
Don't let go of your hard earned money unless you have to!