Ibanez 4-string bass with a pickup for 5-string bass mounted. Why?

Wide string spacing perhaps.

I have an SX 5-string with 4-string size pickups...so who knows.
 
It's probally to make the manufacture of their basses cheaper. Stocking one part instead of two can save money when your mass producing things.

If you pay attention to a car companies models you will notice they often use many of the same parts on several models of cars for the same reason.
 
Thats probably a type-o. Why would they waste extra money, as negligible as it may be when buying parts at cost, on a more expensive pickup for something like that?

If it isn't, the only reason I can see otherwise is because the string spacing is too wide.