J.L.
My daughter is a 4.5 only, as well.
There is such a push lately to do play dates at such a young age. Is socialization important, sure. But she's getting interaction with children five days per week. If she is well liked now, when the time comes for more extra curricular activities, she will still be well liked.
Knowing the other parents is important, from my point of view, because I want to know, when the time comes for drop off play dates, who my child will be influenced by. But I honestly don't have the time to be friends with all my child's playmates and as a working mom, sounds like you don't either.
On some weekends we do birthday parties, vacations I send a general email invite to park/Zoo/Seaworld, but family time, for me, is first.