I have a tom tom xl2, bootloader 5.5274, internal memory 2 GB.
I decided to add an internal SD card of 4 GB or 6 GB if possible.
I wonder are my bootloader supported SDHC memory or not and supported more than 4 GB ?

6GB won't be possible, 4 and 8 ist standard size, even if you find a 6GB I would not recommend it.
Bootloader should be fine if you don't have any USB transfer problems currently.

Thanks a lot.