The radio waves are electromagnetic wave, so it travels with velocity of light i.e .
We can use the relation between frequency, wavelength and speed as
Here c is speed of light, is wavelength and f is frequency and its value is given 99.90 FM, it is actually in megahertz (i.e 99.90 MHz).
Therefore,
.
Thus, the broadcast wavelength of the given radio station is 3.003 m.
The fourth dimension is technically time. the fourth dimension that you are talking about is actually impossible to comprehend.