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Assume the input voltage is a sine wave, there is no appreciable saturation in the transformer, it is an iron core transformer with high permeability, there is tight coupling between the primary and secondary, and the input 115 is the RMS value.
Then the secondary RMS voltage under no load conditions is close to 115/4 = 28.75 .
The peak voltage of a sine wave is the square root of 2 times the RMS value, or under these conditions the approximate output voltage is 1.414 * 28.75 = 40.65 V.
In the real world this is usually less because of some leakage flux that does not include both coils, although this might be small. Because of internal impedance the output will usually be less as load is increased. Appropriate capacitive loading on the secondary could cause resonance and an increase in output voltage.
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