How is space time measured?
s = Spacetime
c = speed of light
t = change in time
r = displacement vector
Space time is often suggested to be fourth dimensional because of spacetime being measured on width, length, height, and time.
To measure the spacetime of an object moving through an environment with no influences you would need the object's displacement , or how far it moved from its original position. You would also need how long that movement took.
Spacetime can also be plotted on a four dimensional graph using the coordinates (x, y, z, t).
Further reading here.