You will gain an increase in torque proportional to your increase in displacement.
The equation for horsepower = (torque x rpm) /5252
So lets assume your 3.7 makes a peak 300 whp @ 6500 rpms. That means you're making about 242 WTQ at peak whp.
So, lets take our 3.7 and increase it to 4.0, that's an increase of about 8%.
So, 242 WTQ x 1.08 = ~261 WTQ. At the same 6500 rpms, that now gives us a peak RWHP of about 323 whp.
So, obviously the more power you have, the more it will help. However, keep in mind drivetrain loss is a percentage as well, so I don't know that there will necessarily be diminishing returns, but it is a factor to keep in mind. Keep in mind also, the RPM at which you're making peak rwhp will effect things as well, I simply pulled 6500 rpm out of my *** for the sake of example.
More importantly however, I would not slap a stock top end onto this motor, this thing will want to breathe, the stock intake manifold and generic CAI will likely hold it back a tad. However, this is all just ricer math until the rubber hits the road.