Funny, I was actually thinking about this today since I attend KSU and just watched a documentary on it.
I'm curious if more student deaths would embolden leftist student organizations across the country, such as Weather Underground. I think it would certainly shift the student movement farther left into more socialist tendencies, more Maoist elements, perhaps even a moderation of the Maoist tendencies within the student movement to encompass more youth. Sharp divides between the peace crowd and the retaliation crowd, furthering fracturing of the movement that would curb the growth of it from a worse May 4th.
The givens would be harsher outrage from the public as a whole. No matter what the students did that might have provoked it, no one with a shred of competency is going to sympathize with soldiers killing 40+. Most likely a permanent removal of ROTC from Kent State, as well as other universities across the country. Overall, a general disdain for the military among the youth that could have a hand in effectively killing the draft entirely. A tragedy such as this could have a monumental effect as the generation matures, initiating greater peace trends.
No effect on Cambodia or Vietnam. Nixon sticks to his guns, and only the resultant chaos at home, and possibly worse incidents that could occur from a greater tragedy would have any sway in his decision making.