I pretty much agree with everything that's been posted above. The SD IV really does have an edge over flatbeds -- doesn't matter which flatbed you're talking about either. But a good Epson or Canon should do a more than acceptable job for medium format.
I was in your situation a couple years ago. I had a budget of $200 and I wanted a better scanner than my Epson 3170. I wanted one that would scan medium format, but my plans have been to buy a large format system someday, so I researched what was available, used and new, that would handle 4x5. I came up with two scanners that fit my budget: the Epson 4990 and the Canon 9950. That Canon is proportedly an excellent scanner, but is about as rare as hens' teeth. The Epson is more common. I ended up doing a nationwide search on Craigslist, using a web-based utility called SearchTempest, and found a 4990 for the right price in Chicago (I live in Houston). The owner agreed to ship, and a week or so later I was the proud owner of a lightly used 4990.
Welp, I still don't own a 4x5, but I've scanned a lot of 120 negatives and slides, and it's done a great job. So, basically what I'm saying is, don't rule out used. The 4990 was replaced by the V7xx models and is closer in design and function to those than it is to the V500 or V600, so you're getting a lot of scanner for a fair sum -- should you go that route.