The Guava Lotus is an easy to check pack n play. My kiddos both slept well in it and the side zipper came in handy a few times for each. Baby Bjorn has a better mattress, though. Honestly, do you think you'll go there often? Do the people you are staying with have room to store one? If so, I'd have them buy one or you buy an extra and bring it there to stay. I finally did that with my mom and it was so nice not to have to cart it every time I visit her. (She also now has toddler plates/cups/silverware, a few toys/books and a potty.) You can pick up a great condition, used Guava or Baby Bjorn pretty easily around DC.
We love our Babyzen Yoyo as a travel stroller, but at 4m, your kid will be too small to use it without the pricey 0-6 pack.
You'll want to wear the baby on the plane if it's a lap infant (so you have free hands at the airport), but the airline will make you remove the baby from the carrier before you take off, as they aren't FAA approved.

Somehow it's better to have a flying projectile of an infant in case of crash instead of not. My husband always helped me hold the baby extra during landing, in case it was rough.
Plan to nurse at takeoff/landing or use a pacifier at those times.
Don't worry--4 months shouldn't be too bad to travel with. The baby won't be mobile yet, so he or she will be content to stay in your arms the whole time. It's 9m to 2 years that is the really tough stretch!