The SP4 will be more useful to you than the SP3 was. Although, the SP3 technically had a larger battery. to get 3 years out of it is " okay ". However I noticed you only had 16 cycles, was the device recently re-installed back to factory settings ? Or did you just leave it plugged in most of it's life?
The SP4 will out perform the SP3, and it seems you got quite a lot of use out of it, so i'd say it's a solid investment.
