I assume that you are considering Mitee-Bite TalonGrip clamps. I have successfully used their clamps for many projects. However, they suggest a grip of 0.060 rather than the 0.030 that you mention. I would not be comfortable with such a small grip. Consider also that most extruded aluminum has a small corner radius rather than a square corner. The rounded edge will further reduce your grip.
Additional thoughts:
You mention that the parts are about 4x4 inches. Could you process them in perhaps triples and separate later? The gap between parts would provide more options for clamping.
Is it possible to cut partially though the stock (perhaps 0.15 deep) on the first operation and finish the cut in the second operation when the part is flipped? This would allow for more grip allowance.
I don't know your location but is 8mm stock available? That would provide ample clamping room.
With 0.25 by 4 stock clamping tightly at the bottom edge will cause the stock to bow upwards. If you facemill the top surface the result will be thinner in the middle and must be considered in the final dimensions.