Is it an older MP40 or the MP40QD? There's a big difference in the amount of flow created... A single MP40QD can thoroughly create flow in a 4' tank, though two at lower settings is always better for creating varied flow.
Anyway, my point in saying that is that an MP40QD will need to be kept VERY low on a 40 breeder. I wouldn't put it past like 20%. I thin the best solution would be to run the MP40 on one side, then an MP10 on the other side and the 3rd MP10 on the back. Make the MP40 the master and run it low at like 20%, then override the max output settings on the MP10s and run them a little higher like 50 or 60%.