I have 4 points defining a curve, I want to interpolate more points
between them. The 4 points have x,y,z coordinates. And the number of
points between is variable.

A great answer would provide a function written in C. 

double*  getCoordinates( int numbetween, double* loc1, double* loc2,
double* loc3, double* loc4)

where the value returned is an array of the N = (4*numbetween -
numbetween) coordinates along the curve.

I found an answer for a question similar to this one, but the answer
was removed. And to put the original asker at ease, I am not a
competitor. I can pay more for a rapid answer.  

Thank you.