In two of the previous times, the President under investigation was impeached; however, in neither of those impeachments was the President removed from office (although Johnson only held onto his job by one vote). The third previous inquiry was the only one to lead to a President leaving office; although, Nixon did so before he was actually impeached (largely because he was just about to be).
In all four cases, the House of Representatives has been under the control of the party opposite the President. In all of the inquiries, the minority party in the House claimed that the proceedings were politically motivated (although many Republicans joined Democrats and voted for the impeachment inquiry to begin for Nixon). For those who enjoy numbers, the steps of impeachment were brought by the 39th, 93rd, 106th, and 116th Congresses against the 17th, 37th, 42nd, and 45th Presidents (I notice the events are happening closer together).
It will be interesting to see how these events will be viewed 31, 45, or even 151 years from now (assuming that we still have years in the future). I'm certainly interested to see what even the next year will bring.