I got the TomTom One New Editon v4 (4N00.005) with 1GB internal + SD Slot.
I was wondering how big SD Card can i put in here, will a 4GB work or do i need to find a 2GB card?
What kind should i get?

2GB

58015800

[b][b] 4[/b]GB
58035802

[/b]

It depends on your bootloader version. Below 5.5012, you are limited to a 2GB SD card. With 5.5012 & above there is no limit. I have a basic SanDisk SDHC class 4 16GB card that I use for testing purposes. FYI, a higher class will not improve performance.