I'm thinking of buying a HMDI to Type-C cable to watch Blu-ray movies on my tv (my tv doesn't support exFAT format and i have a very slow internet so I can't cast my phone to the tv so i have to download the movie on my phone) so my question will the tv play the movie on 4k or 1080p
You probably should look for help in places specified for your phone. This is Android TV subreddit.
That depends on the spec of the type-c port on your phone, you should ask in a sub-reddit for the specific model phone if it isn't declared anywhere.
Well, it depends.
If your TV supports 4K then it will play 4k, 1080p, 720p or lower contents. But, if it supports 1080p, then 4K will be downscaled or just not be played. I have a Samsung HD TV (720p), it supports FAT32, ExFat and able to play 720p and lower from USB or from Samsung S9. 1080p plays downscaled (720p). Try to Google with your TV Manufacturer and Model Name for supporting Resolution and File format/type. If you have downloaded contents on your phone, you may use compatible screen mirroring device (of your TV or add-in like Chrome Cast, or Android TV Box that supports your desired+tv resolution). For Screen Mirroring you don't need a fast internet, it depends on your phone and TV only (if there are not too much wireless interference).
Edit: For DOWNLOADED/LOCAL CONTENTS, Screen Mirroring don't need a fast internet, it depends on your phone and TV only (if there are not too much wireless interference).
There is more to a video than just the resolution
Casting is local, both with Miracast and Google Cast, it won't use your Internet connection to stream the video