The disk should first be formatted using the LaCie setup. I think I saw that EXT is also available, but I'm not sure. HFS+(mac), NTFS(windows), Fat32((not recommended) or ExFat (Windows and Mac) are certainly available. For Exfat, you can install the Exfat package in the Sinology package repository.
If you prefer EXT4, and the option is not available in the LaCie setup, then Paragon disk manager trial via Windows would be an alternative. I think the Lacie Nas should also be able to format the external hard drive in EXT4 when detected. Check the manual.