I created a muti-checkbox form element, and thought I'd share. This was built using version 3.6.4. If anyone has a more efficient solution, please pass it on.

Within the colModel, the appropriate edit fields look like this:

edittype:'custom'

editoptions:{ custom_element:MultiCheckElem, custom_value:MultiCheckVal, list:'Check1,Check2,Check3,Check4' }