Well, if your "pressure" is 4 MPa on both ends, then none. If the differential pressure is 4 MPa, then a really lot if your pipe is really short, not so much if the pipe is long. If the water freezes then none.
This is an Engineering forum. Your posts will receive much more positive attention if you provide the basic information that an Engineer would expect to have to solve the problem. In this case you were missing (1) point where the pressure was measured; (2) pressure at the other end; (3) length of pipe; (4) pipe material (it matters for the friction calculation); and (5) temperature of the fluid (which has a second order effect as long as your dP doesn't take you to a phase change region).
David