Look out for the older Thule 903, carried 3 bikes as standard and you can fit an optional 4th rack. Solid and well made
Take a look at BakRak's
More versatile than a bike only carrier as you can carry all sorts with them.
It's basically a metal plate that you can attach standard bike racks too
I'll dig a photo out if I can
Ok here's one,
Now I've got big Thule bike racks so with the standard BakRak could probably only fit 3, but they sell an extension which they say will take up to 5 bikes
But the good thing is you can use it to transport all sorts of things
https://www.google.co.uk/search?q=bakrak&client=safari&hl=en-gb&prmd=simvn&tbm=isch&tbo=u&source=univ&fir=-8zYK1lpiw3wgM%3A%2Cmfd5hdCcCJ85cM%2C_%3Bd6ruUFzp5q4ivM%3A%2CbK80JyMCQrA4OM%2C_%3BLj2uNArgi8QALM%3A%2CMgxc9K8u3uMeNM%2C_%3B2B5sTeHzLod6JM%3A%2C2renHUgVp6CTHM%2C_%3B--Zfc4-z_xnVPM%3A%2CKKlJykKZb_lNAM%2C_%3BYpTCoMqExfSuRM%3A%2CRdSHkbJSVKslvM%2C_%3BcupiUCf7FJ8i3M%3A%2CMgxc9K8u3uMeNM%2C_%3BLie5h5iOzKvChM%3A%2CZs8R10eAD0g-0M%2C_%3BHKmGvt7A642uWM%3A%2CMW9Bd3FQF5IX2M%2C_%3BlSNd9h0vrit60M%3A%2CQJ054_VxboHjjM%2C_&usg=__tfAKifjL8L5iSIOSEDnQ1Wmo1uc=&sa=X&ved=0ahUKEwiKsbrhi5DMAhWK1hQKHRH-BVUQ7AkIgAE&biw=375&bih=559
Not sure if that link works as I'm on my phone but it's a link to a google image search for BakRak
Looks like a good idea, and a useful bit of kit.
Word of caution though........
Lots of cars have a towball weight limit of between 75 - 100Kg.
That limit applies at the towball, which is a couple of inches from the bumper. Any weight which is applied further from the mounting (i.e further away from the bumper) will have a larger effect.
I guess up to 4 bikes would be OK, but a heavy box for example could could potentially damage the towbar and/or car chassis.