I do put multiple machines in the 4U. I feel sheepish about sharing pictures. My setup is not as trim as I would like. Hopefully my description will give you an idea of what it looks like.
For the boxes, I have a 1U enclosure holding two Mini-ITX motherboards. Then I have a 1U switch, it's nothing special, cheapest gigabit switch that got good reviews. And then for my load balancing and SSL, I got a Kemp Technologies hardware load balancer. It's got a little ASIC in it that offloads the SSL from the servers. I think I can sustain 200 concurrent SSL requests, which is fine for me right now while I develop my app.
In my 1U holding the servers, each server has a two disk software RAID-1 setup. I can't physically get to the colo but once or twice a year max, so I need to be able to withstand a drive failure here or there.
I think I get a drive failure about once every 2 years, and one ram stick failure so far. I had one motherboard failure, but it was a VIA technologies chip/board, and since then I have switched fully to the intel-produced slim Mini-ITX Atom motherboards, and those things are rock solid - just wish I could get 8GB of ram for them for more memcached goodness. ;)
I don't even know if my dual Mini-ITX server enclosure is sold anymore, it's kindof freakish especially with heat. Since I built this box I have been investigating "shorty" or "short depth" 1U enclosures. I wanted to be able to pack in the servers, and you're allowed to bolt servers to both sides of the rack. So by transferring my servers to shorty enclosures, I should be able to spread the heat out a little better and max out my space. I think I have space right now for 3 more shorty boxes if I need to expand my cluster.
Edit: for physical access, you just call the support line and either ask for remote hands/smart hands if it's something simple like rebooting a box for you, or you can schedule a time to come in. I've never been turned away from coming in the same day and when I want to. I am usually alone in the server room when I work. With all the cooling equipment and servers, it is very loud in there. Sometimes I just wear earplugs to dampen the noise. They've never charged me for smart hands, but I don't ask very often, once a year maybe.