I don't know much about components on bikes like the Venture, but my guess is that the 4.0 might be a bit lighter, and more importantly is an 8-speed bike, whereas the 3.0 is a 7-speed. This will probably matter most on hills, especially steep ones. If you're not going to be riding the Venture (or similar bike) on hilly terrain, I think it would make sense to save some money on that purchase so you'll have more available for your road bike (and diapers!).
As for buying two bikes vs one, I'm actually a bit of an outlier around here because I don't believe that n+1 is always the correct number of bikes to have (where n is the number of bikes you have now). You should buy a bike if you can afford it and you expect that you will ride it. If it (or a bike you already own) just sits and collects dust while you ride something else, you're probably better off saving your money.
But you seem to have a plan for riding the Venture (or similar bike) with your kids after #6 is born, so it might very well make sense for you to have both that bike and a road bike later on. And depending on how old your other children are, you might end up handing down the Venture to a daughter in a few years.
BTW congratulations on #6!!
- Gray 2010 carbon WSD road bike, Rivet Independence saddle
- Red hardtail 26" aluminum mountain bike, Bontrager Evoke WSD saddle
- Royal blue 2018 aluminum gravel bike, Rivet Pearl saddle
Gone but not forgotten:
- Silver 2003 aluminum road bike
- Two awesome worn out Juliana saddles