Rule number one:

Do not use United.com for anything. Transfer your miles over to Continental, use Continental's award search tool and enjoy the better experience

Seriously, United only searches for UA/CO flights, as well as US Airways and Lufthansa online. Continental's website searches all Star Alliance carriers (except for Swiss), and also works a lot better at computing routings too. Chances are you will probably find something for four seats (it's sometimes even possible to find 8 seats in business class on the same flight, transatlantic), so just keep looking and you will probably find something. Searching segment by segment may be more effective, but can be also time consuming.