What is the output voltage if an input signal is applied to the secondary winding of a 4:1 voltage step-down transformer instead of the primary winding?

A transformer consists of two windings. A '4:1 step-down transformer' means the primary has 4 times as many turns as the secondary, normally dividing the voltage by 4. If you reverse the normal operation by applying the input signal to the secondary instead of the primary, the transformer acts in reverse. The secondary becomes the new 'primary,' and the 4:1 step-down becomes a 1:4 step-up transformer. Because the step-up ratio is 1:4, the output voltage taken from the original primary winding will be multiplied by 4.