Great thread! To start, I currently own a Squier 70's Vintage Modified P that I absolutely love to play. I bought it because I love the P Bass sound, but didn't want to invest a lot of money into a bass I wouldn't play often (I'm addicted to the J-Bass sound). I know people have a negative opinion of the Squier line, but they have changed a lot of minds with the introduction of the Vintage Modified Series (VM P, VM J, and VM Fretless), the Classic Vintage Series (Classic 50 P and Classic 60's J).
I started on a 4 string, but soon bought a 5 string. At first, I had a problem switching back and forth, due to different string spacing. Then I found a Yamaha RBX 765A and my problems were over. The string spacing matched my 4 string, so I stopped "missing the strings. here are some of the things that I've experienced so far.
1. By using the B String as an anchor, I improved my e string technique
2. That practicing 2 octave scales really tremendously help with finding the best position to play runs with minimal movement up and down the fretboard.
3. Doing my woodshed exercises on the 5 string make playing the 4 string much easier.
4. Having the extra real estate that the B string provides is a lifesaver. Have you ever started a run on a 4 string, feel that you want to go deeper, only to realize you've run out of fretboard?
5. Unless you are really good at keeping two sets of fingering in your head, you should commit to learning on either the 4 string or the 5 string. You already have enough things to focus on.
6. With all this in mind, I would focus on finding a 5 string that would serve as your workhorse. This way you have the option of playing it like a 4 string, then switching to playing all 5 when the time is right. However, if your like the rest of us, you'll be gassing and end up with a bass for every situation.