Grooves in rotor and grinding noise with 4mm brake pads *with picture*

About a week and a half ago I noticed an intermittent grinding noise near the front driver's side when using my brakes. One week ago I took my car to the dealer to get the noise checked out and have an inspection and oil change. When I picked up the car I was told my brakes were fine, the front (disc brakes) were at 4mm and the back (drum brakes) were at 3mm.

The noise continued and I had my dad take a look and he noticed that my left front rotor is scratched with grooves in it, whereas my right front rotor is totally smooth. I took my car in again today, and the service guy basically said that my brakes are safe since they are at 4mm, and that I can either keep driving on them with the noise, or get everything replaced for $500. He said those were my only two options, and that it would be a little early to replace everything since I had 40% of my pads left. I asked him if I didn't get it fixed right away, could it cause more damage and end up costing me more and he said he couldn't give me an answer. I was a little flustered at the time because he was acting like they had already explained this all to me when I picked up my car last time, and they hadn't (I had only been told by the cashier that my brakes were fine), so the whole thing ended up being a wasted trip. I left confused, with a bad feeling about the dealership. I'm having a hard time believing that it's safe to drive when I keep hearing a loud grinding noise when I brake, especially because I am a new mom driving around with a 6 month old baby. If you could give me any information or advice, I would really appreciate it. I would mainly like to know if it's safe to drive around like this, and if the price of $500 is reasonable to pay when my brake pads are okay and only one rotor is scratched and making noise. I've included a picture of the scratched rotor.