The first thing we need to establish is whether or not the velocity in the suction line needs to be maintained above 1.5 m/sec. This will depend on the nature of the slurry.

If we really do need to maintain the 1.5 m/sec. velocity, then. . .
In a 3 inch suction line, that velocity will be maintained if the flow rate is maintained above 110 USGPM
In a 4 inch suction line, that velocity will be maintained if the flow rate is maintained above 190 USGPM
The industry standard sizing for a suction line on a pump with a 4 inch suction nozzle, would be 6 inch diameter, at which size the flow rate would need to be in excess of 440 USGPM in order to keep the velocity in that line in excess of 1.5 m/sec.

Further advice on pump piping can be found in my article "The Pitfalls of Pump Piping" on my website www.practicalpumping.com/articles.php

As the discharge piping is normally one size smaller than the suction piping, I can't help but wonder what size the discharge piping may be in your installation?