The Video Toaster 4000 capabilities were delivered as both a hardware and software upgrade to the original Video Toaster. Hardware wise, the video processors were improved on the 4000 to include many new digital video effects and faster performance, as well as to take advantage of the enhanced graphics capabilities of the Amiga 4000. In order to achieve this, the VT4000 connected to the A4000 using the new Extended Video slot. This gave the VT4000 access to additional video signals not present on the A2000/3000 Standard Video slot. See video slot pinouts here.
Since the Extended Video slot is a superset of the original slot found in the A2000/3000, the VT4000 is also compatible with the earlier machines. However, many of the video effects and performance improvements of the VT4000 will not be available. You need an A4000 to use the full suite of effects and software.
The VT4000 does not take advantage of the inline Zorro-III slot that the A4000 has. Communications between all Amigas and all VT's is through video slot signals, which includes an 8-bit parallel I/O capability.
There was no size advantage to the VT4000, and it has pretty much the exact same form-factor as the original. As such, it would actually occupy two slots in an A3000 or A4000.