We've taken our baby trend sit n stand twice, once dd 17 months and ds 6 and the other dd was 2.5 and ds 7. It worked great for us. We won't be brining it back this year because ds is just too big for it now, but it worked perfect for our first trip and pretty good for our last one. I think with the ages of your kiddos, it should work pretty well- the only thing I'd be concerned about is the comfort/sleeping ability for your 4 year old. It worked great for me when my ds was 6 (and he even found a way to fall asleep in it one night,) but I'm thinking at 4 she may get more tired and want to actually sleep in the sit n stand, and I'm not sure how that would work. That would be my only concern. For us, it was a great way to get around and have both kids "secure" when we needed to get somewhere quickly and especially at closing when it was crowded and dark.
ETA: We only used Disney transportation, so we had a lot of bus use, and it was fine for us. It's not fun to take any stroller on a bus, but I didn't find getting this one on and off much more difficult than when I brought my regular full size single stroller. It collapsed to be just a bit larger, and it was heavier, but still doable.