Tough questions

My daughter asked me when me, her daddy, and herself would move back in together.
How do you explain to a 4 1/2 year old that it won't happen? How do I explain that for reasons she can't understand that it's for the best? How do I make her okay with the answer "never"? How do I explain that we aren't moving in together? That The separate house thing is permanent?
She talks about our old city a lot. It's where her world last made sense. Her Parents were together, she could see my mom (her grandma who's lived within 5 minutes of us since she was born) whenever she wanted, and her Jazzie cat was there.  She loved her Jazzie cat who died just a few months before we moved from a stroke.  She knows that Jazzies no longer in Asheville, but in her mind, all is right in the world there.  This makes me sad on so many levels. Mostly because I don't want her to associate this city we now live to be the reason her parents aren't together, that her grandma isn't a short drive or walk away, or why she no longer has the coolest cat in the world.
This is tough. You always want to protect your kids, to make the right decisions.  But sometimes what's "right" is hard to grasp.