I want to upload 4 portfolio images at the same time but only one get's posted

Yes that is right. As the Add Portfolio Image form already says, you can only upload one portfolio picture at a time. When you click the SUBMIT button you can go back to Add Portfolio image in the menu and upload the next picture. Somewhere in the future this will be improved and will you be able to upload multiple images at one time.

So for now just take your time and upload them one by one.