Newbie Tire Q: ok to replace only rears?

[BMV///M3]

I should probably know this but I don't. My 2016 has 18000 miles and I'm starting to notice tire noise more and more. The Penny test shows what I think is plenty of tread on the fronts (see pics 1,,4). But its looking like the rears are getting close (see #2/3 pic). So like the title says, would I be okay replacing only the rears? I'm planning on replacing with the same stock Conti's.
Also it's important to note I had to patch the left rear due to a nail about a month ago. I had a feeling new tires were coming soon so I didn't opt to replace then. It could be placebo but I feel like since then the road is much noisier, I can hear the tires and road noise more and more.

Please don't interpret the question as my not wanting to spend the money for all 4. That is not the case at all. I'll do whatever I need but I don't want to waste money either.

Thanks

[gatorfast]

No pics showing but yes you can certainly just replace the rears if the fronts have plenty of tread left. At 18k miles I would say you more than got your money's worth on them.

[FriedPiston]

You can definitely replace the rear tires only. Some owners have opted for a square wheel/tire setup to make tire rotations possible, and to help prolong the life of the tires. So no, you're not alone in the money saving category.

[BMV///M3]

. Rears are 4/32 and fronts range from 5-6/32. With the winter coming I'm thinking of swapping them all out for the pilot sport 4s