the 4 points don't have to make a square. it could be an irregular shape which requires all 4 points. also, the grand line isn't a straight line, it has a lot of width and so you can't just travel straight till you find it, which is impossible anyway, because the only navigation you have is the triple log pose thing, which only goes to the next island. I doubt raftel can be reached by the triple log pose thing because it probably has no magnetic field but I'm not sure about that.
you cant go into a straight line in new world sea but yeah there is still chance to get to raftel with 3 but its safer to go for 4 cause of the unpredictability of the sea, well thats what i think in common sense point but we all know its for the story .
The thing is like somebody would have already found Raftel at this point why do you need the ponygliphs really you can find them by chance right its not like the island is in the water like fishman island right