Not sure if this has been answered. I stumbled upon this forum item when trying to answer the question myself - how to print on C4 envelope. A little bit of legwork for myself and resolved it. The solution is already in OOo / LibreOffice, no need for extensions.
Within your document - right click - from 'drop-down' select 'Page' - from this pane the section headed 'Paper format' select 'Format', scroll down the options available and you will find a selection of envelopes. It contains all the the more common or garden varieties :
DL, C6, C6/5, C5, C4, #6 3/4(personal), #8(Monarch), #9, #10, #11, & #12
Hope this helps