You can use the internal function `Internal`RationalNoReduce`

    Internal`RationalNoReduce[4, 8]

![enter image description here][1]

    Internal`RationalNoReduce[4, 8]^2 + 5

![enter image description here][2]


  [1]: https://i.sstatic.net/d2Br9.png
  [2]: https://i.sstatic.net/6KEQE.png