Remember that the sum of the three interior angles of a triangle is 180°.
Now, let's call the remaining angle x. We would have:
Solving for x,
From this, we get that the remaining anlge has to measure 50°. Now, as we already have a side that measures 4 feet, the other two sides are constrained; neither of them can have any measurement that's different to the one that constructs the remaining 50° angle.
Therefore, only one triangle can be constructed.