1. IT Project+ is more of an entry-level project management cert, while the PMP is a mid-level credential with more demanding education, preparation, and training requirements.
2. & 3. Yes, they are in that indeed IT-Project+ is a good stepping-stone to obtaining PMP (and because IT-Project+ requirements are far less stringent than those for PMP--see for example http://www.comptia.org/certification/itproject/q_and_a.asp vs. http://www.pmi.org/info/PDC_PMPHandbook.pdf, which puts the former at about half the experience level of the latter).
4. Approaches are a little bit different, but not so much so that what you learn for IT-Project+ won't apply to PMP.
This was first published in January 2004