Best Way to Convert 4S Lipo voltage to 13.8v SLA power?

[lincpimp]

A123 cells sound like exactly what you want. 3.3v nominal 3.6 fully charged. 1000 cycle life or so. Since it appears you will be charging them with a 12v style charger they will not reach full charge in a 4s config, but will likely last longer.

I had planned to make a 4s3p pack for my motorcycle, since the charging system should not exceed the max charge voltage.

Plus a123 are much more robust and can handle lower discharge better than lipo. Only downfall is weight and the round cell dimension. Neither should be an issue for your purpose.