Hi guys,
i bought an TP-Link WR845n V4, but searching in router database i can't found a firmware for this model, Questions: I can use an WR845N v3 firmware?, view this article
Welcome!
You shouldn't flash an image that's not intended for the exact revision of your device, unless you know what the differences are and you know it will work. The device isn't on WikiDevi and I can't find its specs.
You could try to get in touch with @Heinz to see if he has an image for your device.
I also see this post Wireless failed frequently when using OpenWrt 18.06.1 where it appears that the poster flashed WR845N V4 with WR842N V3 image (after modding RAM and FLA?), and he had problems afterwards.
You could ask people and see if you get the reply you are looking for. But my advice is to not risk bricking the router and wasting your time. It's easier to find a well-supported router in the router database.