Clarification: It wasn't Air Force One that visited Switzerland, it was the 747-200 E4 carrying Ashton Carter. Source
The aircraft is the Boeing E-4B Advanced Airborne Command Post 'Nightwatch', which serves as the survivable mobile command post for the US National Command Authority. The aircraft are based on the same Boeing 747s as the VC-25s used by US President.
There are four identical aircraft performing this function in the US. The "bulge" on the top is a radome that houses a steerable SHF SATCOM antenna, as can be seen in the following image.
Image from cnet.com
The following image shows the aircraft systems in a little more detail.
Image from http://everythingisahoax.tumblr.com
As a side note, Air Force One refers specifically to USAF aircraft carrying the US President. From US DoT ORDER JO 7110.65W
For civil aircraft, it is Executive One.
That is not simply an E4, but a "B" variant. The hump houses a SHF antenna. (Source)