if you have water flowing through a 6 or 4 inch pipe what would the pressure be if tapped off through a 1/2 inch BSP tap. What would the equation be to find out the pressure at different flows?
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You're missing an important part, without which the problem cannot be solved. Is the mass flow rate through the two pipes the same?
In other words, just because the pipe gets smaller doesn't mean the velocity changes in the event the flow rate isn't constant.
However, assuming it is constant, the answer to your question comes down to two things: the mass flow rate definition and static pressure.
The total pressure is the sum of static pressure and dynamic pressure assuming an isentropic flow. So given a known total pressure, changes in velocity result in changes in dynamic pressure which can then be used to find the change in static pressure.
The velocity change as the pipe size changes can only be know if the mass flow rate is known for both pipe sizes (and may be constant).
Those should be enough hints to get you going. Look up the definitions of:
if you don't know them.