Cyberlimbs start at a base of 3 for Str, Bod and Agi.
If you choose Standard cyberlimbs, you have to buy increases to the attribute with cyberlimb enhancements. The enhancements add their rating to the base three up to the Augmented Maximum for you PC's attriibute. So you pay nuyen and capacity in the limb.
If you buy Customized cyberlimbs, they start at three and you simply pay extra nuyen (and raise the availability) to increase the attribute up to your natural racial attribute maximum, then you can go beyond that by purchasing enhancements just like standard cyberlimbs.
In your example, if you purchased a Standard Cybertorso, you would start a Bod 3. If you bought R4 Bod enhancement, it would cost 800¥, take up 4 slots in the torso's capacity and would be Availability 12R. This would give you a Bod of 7. If Customized, you'd start at Bod 3 and then spend 6,000¥ and increase the Torso's Availability to 16R to add 4 to the Bod, raising it to a Bod 7. The Customized torso would NOT be available at character creation since the Availability is over 12.