Situation 2 and 4 can result in the block moving at a constant velocity.
Work Step by Step
To move at a constant velocity, a moving object must experience either no force or equal and opposite forces (this prevents acceleration). Block 2 experiences two forces that are acting opposite of each other, so with equal magnitudes the moving block will travel at a constant speed. Block 4 experiences 3 forces, however, the resultant force of F3 and F2 acts directly opposite of F1 so, with equal magnitudes, block 4 can travel at a constant velocity. Blocks 1 and 3 experience forces that are not opposite, so no magnitudes of force can produce a constant velocity.