Can a 4 gigabyte micro SD card be used or higher the higher gigabyte cards up to like 8 gigabytes are cheaper and easier to find I wonder if the ROKU is smart enough to format it to use just what it needs and ignore the rest of the space.
Yes should be fine. Users seem to have reported that 32GB appears to be maximum size so 4-8 will work just fine for you if your device has an SD card slot.
However, if you have the 4800 Ultra as pictured in your signature, you won't have a SD slot. (Discontinued after the 4670). If using another model of Ultra, then yes, should be good with an 8Gb.