If you use the squashfs-combined version, OpenWRT will run as it does on embedded devices, entirely in memory, only writing when you save configuration changes.
But even with the ext4 version, OpenWRT puts VERY little wear on the card. Logging, for example is still done to memory. You really don't have anything to worry about.
I used a Pi with OpenWRT as my router for about 6 months, and because its storage size and performance demands are so slight I felt comfortable using an ancient 1GB card from the mid-2000s that I'd originally bought for an old flip phone; so old they were still calling it "transflash" instead of microSD. No problems whatsoever, no noticeable effect on boot time and since writes are tiny and rare, no affect on performance.
Another option with the PI4 is to netboot -- I.E. run with no card at all, if you have a fileserver that can do tftp and nfs. I do this with several Pis but not with the router, because a router should have as few dependencies on other systems as possible; if only to avoid circular dependencies. (I did however power it with a PoE hat, which is a very stable and reliable power source if you already have a PoE switch.)