It should be ext4 for Ubuntu, and ntfs for Windows. ext4 partitions cannot be read on Windows, but both ext4 and ntfs can be read and written from Ubuntu.
My suggestion would be to create native ext4 partition for Ubuntu, and store shared files on Windows ntfs partition.
Current setup for my dual boot laptop:
Partition O/S Mount point
ext4 Ubuntu /
ext4 Ubuntu /home
ntfs Windows c:
ntfs Windows d:
As you already realized, drive D (4th partition) is shared between Ubuntu and Windows. I have added it to
/etc/fstab so this partition automatically mounts to
NB: Since this question is not about optimal partitioning, let's skip all the talks about how to partition for Linux.
You want to keep your home directory so your ubuntu settings (.bashrc/various configs) are independent of your install, that way, if you bork your ubuntu, or ever just feel the need, you can just re-install ubuntu while keeping your settings. The reason you want another NTFS partition is because, as many people nowadays, you carry around lots of platform non-specific data (i.e.: movies,audio..etc) putting all that sort of data there enables you to get at it from either boot environment.