If you book the last two segments separately as multi-city so that your e-ticket has four "coupons" then it will be four segments. if you book it such that there is one flight number and one segment for the BWI-ATL-GPT flights, then it will only be three segments.
There was some discussion on this a few months ago.
How am I to know? I am a mere mortal and Egencia doesn't do a really good job of explaing this. I didn't use a Multi City Option, just a round trip option and this is what came up. I'm in no rush to get home and it would be fun, but If I'm only netting 3 segments rather than 4, then whats the point heh?