Because the base has a right angle and side 4 with a hypotenuse of 5, the Pythagorean theorem of a^2+b^2=c^2 (4^2 + b^2=5^2, 16+b^2=25) tells me the missing side is 3. Then, calculate the volume of a full rectangular prism l•w•h, or 8x4x3, which is 96. Divide that in half to find the volume of the triangular prism (96/2=48). Because no units appear on the diagram, the volume is 48 cubic units. Peace!