If you're conmfortable installing EEPROMs, you can try to find a 3rd party seller, but if you want warrantied installation, you're pretty much stuck with dealer pricing.
The V4 offers 4 enhancements:
- THX Surround EX / DTS ES
- 2-second timeout on the front panel display
- Increased A/V Sync Delay for some projectors that required it
- Improved operation of Bass Enhance with DTS
Of these improvements, I find the 2nd one most valuable because I don't like equipment light distractions.
EX and ES would be more useful for a system that lacks other 7.1 processing, but after experimenting with EX-encoded discs I find I prefer Logic 7 anyway--it correctly steers the matrixed rear content and provides stereo rears to boot.
Therefore, my advice would be to wait for the next upgrade, which I suspect will include Pro-Logic II and other enhancements, and will be offered at a deep discount for the first 6 months or so. Sorry, I don't have a timetable, but since you already have Logic 7 I don't think there's any real rush.