I'm not convinced that $4 is the ceiling. We've seen what Relic of Progenitus has done. If an uncommon with several printings can be $5 or more, a rare land with only one printing most certainly can! It could be $7 or even $10 at some point. Plus, this is a Desert from the Hour of Devastation cycle, so the reprint risk is lower than many other lands – although it could end up in a Commander product at some point.
Beyond the possible Commander deck reprint risk, the other risk here is that it hovers between $3-4 and doesn't really become a really good investment. Still, the demand seems to be there even if it seems that the price is plateauing. There is definitely a lot of Standard demand, since you can sacrifice any Desert and not just itself. But it's pretty good in Modern, too, especially as a colorless land im Eldrazi decks. The Eldrazi demand alone can fuel prices as we have seen I'm high on this card and definitely want to keep a playset on hand.
As with a lot of Magic real estate, lands tend to be good investments if you buy in at the right time. This is a case in which I'm looking for a good deal on eBay or something below market price. While I highly doubt people will dump these due to Modern and Commander utility, it may not break $5. But, being colorless means it fits into any Commander deck,.
Also, with the new 60-card singleton Brawl Commander variant being a thing, Scavenger Grounds will be an important card for awhile. I think this is going to be a card that Brawl will cause to skyrocket while it's Standard-legal and I can't see people firesaling these at Standard rotation time in October 2018. Scavenger Grounds looks like some solid real estate if you can get it at a fair discount.