TinyMCE 4 remove/add attributes for all images

Assuming your TinyMCE4 container has an id of ‘tiny-mce-4’, you could add this js to your functions.php.

function toArray (collection) {
    return Array.prototype.slice.call(collection);
}

var imgs = toArray(document.querySelectorAll('#tiny-mce-4 img'));


imgs.forEach(function(img){
    img.setAttribute('style', img.getAttribute('style').replace(/((height:).*?(px;))/g, 'max-width: 100%;'));
});

<div id="tiny-mce-4">

  <img src="https://wordpress.stackexchange.com/questions/207673/img1" style="height: 500px; width: 800px;" />
  <img src="img2" style="height: 500px; width: 800px;" />
  <img src="img3" style="height: 500px; width: 800px;" />
  <img src="img4" style="width: 800px;" />

</div>

  <img src="img5" style="height: 500px; width: 800px;" />