These batteries will be connected in series to give the 48 Volts. It is desirable (mandatory?) that they all be the same capacity. The 17Ah is just a suggested size. The number of Ah (or Wh = Volts x Ah) determines how far you can go on a single charge. So if you had 10Ah batteries, you wouldn't have as much range as with 17Ah.

However, I would really question whether you want to lug around 4 x 17Ah SLA batteries on your bike. I would be looking at a 10Ah LiFePO4 pack - which will be much lighter, smaller, and will last much longer with perhaps a slightly lower range on a charge than the 4 x 17Ah SLA's.