With the idea that you could remove four "concepts" and all related spells from the game and most dramatically cause the game to be altered, I would propose the following:
Healing (the game would have to be re-worked statistically)
Detection (skills would need to be redeveloped/redeployed to move the game forward)
Dispelling (this is a great equalizer, and is often used at higher levels)
Instant Kill (again, a high level way to pick off a target (or targets))
I didn't include "raise dead" or teleport as I've made alterations in my own campaigns to both of these, and it's impact has not been that significant.
If there were four spells to have the greatest impact:
1) Flamestrike - this would cripple the mid-level cleric's arsenal
2) Anti-magic shell - this would cripple the DM!
3) Commune - there's very few spells that do so much, so well
4) Cure Disease - wow, that would be huge, especially to the lower level game