I'm relatively new in manipulating mdraids and LVM manually (I put some time into it when upgrading the Synology, to learn about what happens when the disks are replaced and how their "Hybrid Raid" works on a lower level), but I think a md2+md3 setup (and ext4 directly on these) is the best way to go.
Doing so on the same disks will not waste any space, and will probably give a little bit of better performance:
md3 : active raid5 sdd7[4] sdc7[2] sdb7[1] sda7[0]
11699606016 blocks super 1.2 level 5, 512k chunk, algorithm 2 [4/3] [UUU_]
resync=DELAYED
bitmap: 0/30 pages [0KB], 65536KB chunk
md2 : active raid5 sdd6[4] sdc6[2] sdb6[1] sda6[0]
11699606016 blocks super 1.2 level 5, 512k chunk, algorithm 2 [4/3] [UUU_]
[>....................] recovery = 0.3% (11845376/3899868672) finish=2773.6min speed=23361K/sec
bitmap: 0/30 pages [0KB], 65536KB chunk
Here each partition on the four disks are 3.6TB (same size in both arrays), so I will waste a total of 7.2TB for the parity. The same would have been wasted with four partition of 7.2TB, so no difference at all.
Btw. Thanks for your guide on how to get Debian on the NAS. Really easy to follow once able to put the files on the device.