The default for my Synology DS415play is ext4 so I was wondering if the BDP's can access drives formatted in ext4 without issue or should I look at formatting in something else, perhaps NTFS? I want to avoid Fat32 if I can due to it's 4Gb file limit.
I believe the BDP with latest firmware/OS is ext4 compatible, so this might only be a concern if you were directly connecting an ext4 drive to the BDP and also needed to rip to it from a non-Linux computer. But because you are accessing it through a NAS, the NAS' Linux OS is compatible with ext4 and becomes the intermediary between your computer or BDP device, regardless of the OS, and the drive.
The only incompatibility might be if you needed to access the drive removed from the NAS and directly connected to your Windows or Mac computer. These OS aren't natively compatible, but can access ext4 drives with third party drivers/software. Of course if the drive is in a RAID configuration, that might be a moot point anyway.
Steve