replacing low e with a b

Yes you can, but getting a four string set of heavier gauge, like the Dunlop Heavy Core (there are others, I just can't think of any at the moment) would save you from paying for a string you don't use. You could also buy singles and create a custom set from www.bassstringsonline.com

 

BSO is probably your best bet, then.

 

If you go to high up in the gauges you may need to enlarge the nut slots to accommodate a thicker string. I run my 4 strings with .124s and several of my basses are fine with minimal adjustment. But most E strings on basses are around .105-.110. Just keep that in mind when you're picking a gauge.

 

Ibanez AFB 200 is a short scale bass at 30.3".
If you use regular strings, with a typical E somewhere around .095 to .105, they will be loose since they are designed for a 34" scale.

A typical E string on a 34" is about .102, at about 40 lbs of tension.
A 40 lb. balanced roundwound set for a 30.3" scale 4 string bass would look something like:
E .118 42.22 lbs.
A .086 40.71 lbs.
D .063 39.24 lbs.
G .047 40.84 lbs.

That's based on the unit weights published on the Kalium (Circle-K) website. Different brands will have slightly different unit weights which will throw the numbers off some, but the above should give you a good ballpark. I have a spreadsheet I made to calculate tensions & gauges at different scale lengths.

Whatever strings you get you want to make sure they fit your bass length wise. If they are tapered you need to make sure the taper ends near the bridge saddle and after the nut and before the tuning key.

Hm I just read that you are looking for flatwound strings. The numbers above are based on roundwound strings. Flatwounds will probably have a higher unit weight. If the string manufacturer publishes the unit weight for their gauges, those could be plugged into the spreadsheet and you could see the tensions.