How to split taxonomy checkboxes on node edit form I figured how to do it on node output, but i can not figure how to add css for specific field, in this case taxonomy checkboxes, 50 of them, and split in 4 columns on node edit form.
You can do it with css. You will need to add the following to your Admin theme's css file. 

The Drupal default admin theme is Seven theme, located in /core/themes/seven. You may want to create a sub theme of Seven and use that as your admin theme, so when you upgrade Drupal your css file does not get overridden; thus, you will not lose all your changes.   

#edit-field-name {
  width:760px;
  margin-bottom:20px;
  overflow:hidden;
  border-top:1px solid #ccc;
}

#edit-field-name div {
  line-height:1.5em;
  border-bottom:1px solid #ccc;
  float:left;
  display:inline;
  width:25%; }


replace -name with the actual name of your field. 

Source: http://csswizardry.com/2010/02/mutiple-column-lists-using-one-ul/