J.G.
Cali has nice weather, right? LOTS of great parks y'all have there (in Anaheim at least - I went to two parks there with big open spaces and picnic areas.) So a park might be a good idea.
I agree to keep things neutral between you and her friend, have the location be near the prego-M.. Or near her grandparents/family so that the elderly don't have to travel far.
Big sis -- a t-shirt or some sort of 'big girl' toy would be great. Is the new baby a boy or a girl? If a girl, maybe the 4 year old gets the "HEAD PRINCESS" shirt or a crown to wear. Maybe a doll with diapers and clothes so that when mommy's changing the baby, she can change her dolly too.