I'd wait for the 4000 to arrive and try using your monitors. I'm not familiar with the Spectrum card, I'm sure others are, but it might handle the video for you.
The Scanmagic would be a good alternative, but I'm sure that Softhut has long sold out. They are hard to come by. I just sold one on auction that went for $150.
Individual Computers is making a new scandoubler/flickerfixer for the A1200 that looks to be miles above the current old units. An A4000 version is in developement. Haven't heard a release date but, hopefully, it should be next year.
Good luck with your A4000. You might want to take a look on the message base for information on battery leakage. Hopefully you won't suffer that problem.
Good luck!!
Bob