Well, the 4GB limit as described above applies only to the internal IDE driver of the Amiga. If you have a third-party IDE controller like the Elbox FasATA or the Buddha Flash which allows to use 64bit commands without any patch, you can create one big partition on a HDD of any size up to 2TB (2'32 * 512).
Physical block size is still 512 bytes and AFAIK both IDE and SCSI refer to 512 bytes blocks by 32 bit block numbers, so 2TB is the physical limit until someone expands the SCSI norm.
Just for correctness, the IDE driver is called scsi.device, not trackdisk.device. OS3.5+ does not change anything on trackdisk.device, it replaces scsi.device.
So, in conclusion, if you boot from the internal IDE bus, your boot partition has to be smaller than 4GB and *it has to reside below the 4GB limit*. People usually don't see that the 4GB limit applies to the entire HDD, not only to partitions.
Bye,
Thomas