This is most likely the case, as I've told you before when you've stated "there's no need to get them all". They don't make a beautiful perfect cross, so not even 3 points are enough, because you don't know which 2 of those 3 form a line and witch one is the dot that is connected with the unknown 4th point. Try and draw 3 dots on a paper, you'll see that depending on when you add the 4th point the possibilities are nearly endless.
Add to that that this is not just a sea with islands, it's a world cut in half by a long piece of land who can only be crossed through specific points (Red Line), and cut in half by a sea which is nearly impossible to navigate through (Calm belt). Therefore, even if you had an idea, it would be impossible to walk a line between two islands straight, as these lines could (and most likely will) cross either the red line or the calm belt. And you don't want to just skip those points either because who knows, Laughtale could be in the Calm belt or on/under the red line. Therefore... yes you need the 4 of them to work it out.
I am sorry I think I see the problem here. For the longest time I have been assuming there is the name of an island and the direction. Now if you know anything about maps (why would you? Why would I? Dont ask...) you know there is a simple technic called intersection. You get two known points and find the direction to an unknown point from the two known points and you have the coordinates to that unknown point.
So I understood oda to be saying "the road poneglyphs give an island and direction to laugh tale"
So my mind went to intersection. My first thought was "well shit, I only need 2 known points and directions. Maybe 3 because this is the NW and navigation is supposed to be hard..."
So what you are saying is that there is no direction at all? And the red stones just give you 4 islands and you have to do some math to figure out where the center of them is?