When you say 1 over 3, you are basically saying 1 divided by 3, or expressed as a rational number: [tex] \frac{1}{3} [/tex]. in our rational number the denominator is 1, and the numerator is 3
Now, a rational expression is an algebraic expression in a rational for. If we apply the same to your problem. 4 over x-3 means 4 divided by x-3, or in mathematical notation: [tex] \frac{4}{x-3} [/tex]. In this case, the numerator is 4, and the denominator is [tex]x-3[/tex].
We can conclude that the equivalent rational expression of 4 over x - 3 is [tex] \frac{4}{x-3} [/tex].