I have successfully used 2 vouchers to upgrade 2 people on a return, but the agent implied that 4 people one way would also have been possible. He said it's "not strictly allowed" but had no problem putting it through.
There is a back office process they need to do for this which was missed in my case resulting in the reservation not being ticketed. I rang back to query this and it was immediately sorted, but do check your confirmation email has a ticket number listed.