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USB 2 vs USB 3 is a non-issue in your case. Your TV's USB port is almost certainly a USB 2 port and will treat a USB 3 device as a USB 2 device. Also USB versions don't place limits on device size, file sytem or file size, or the file system you can format the device with.
The real question is what file systems does your TV support? Since you're already using a 4 GB device then you're probably using FAT32. Unfortunately Microsoft OSs limit the creation of FAT32 file systems to a maximum size of 32 GB. The are many tools that will format a FAT32 file system larger than 32GB (and modern version of Windows will have no problem reading/writing them), but without knowing what TV you have there's no way to know if it is one of them.
So a 64 GB flash drive or even a larger hard drive (as long as it's not a bus power only device as most TVs don't supply enough current to power a hard drive) should work fine with your TV, but you may have to format it using something other than your TV. I can't say for sure since you didn't tell us what TV you have.
The more important limit you may face is that FAT32 files are limited to maximum size of 4 GB. if you want to watch files larger than 4 GB then your TV will have to support a file system like exFAT, NTFS or a Linux file system. But, again, there's no way of knowing what file systems you can use since you didn't tell us what TV you have.