Back in the fiction saddle

I've recently gone back to my fiction writing after almost ignoring it for the last six months. I never really stopped writing (worked on stories here and there), but family demands didn't leave much time for anything other than family demands.

Now that my five-month-old daughter has a set bedtime routine, I'm able to find time at night to return to my fiction. I basically have four open projects:

1. A fantasy novel -- set in a world with 19th century technology --that is already written, but needs heavy revisions.
2. A contemporary mystery novel starring a female computer hacker from Hamtramck, MI. The outline is done, and I'm a quarter of the way through the first draft.
3. A hardboiled sci-fi mystery set 400 years in the future. Same progress as #2.
4. And a sci-fi/horror novel that's like Night of the Living Dead meets Aliens. I have the basic plot points down, but I'm still not through with the outline.

The question is, which one should I concentrate on? I love all four stories, but I can't seem to focus on one at a time. I work on one for a while, then get great ideas for another one and start working on that one...until I get great ideas for a third one. I know I can complete a book from start to finish -- I've done it before -- but I'm having trouble lately focusing on one book at a time.

So is it a bad idea to work on four different fiction projects at the same time? Does it make you lose your "voice" for a particular book, or does it keep you energized by mixing up the worlds and "voices" you write about?