Why don't you just use 2s LiFe pack? I don't see what the big deal is, the peaked voltage drops down to ~3.3v pretty quick even if the load is small. I wouldn't worry about the peaked voltage, it's not going to blow up.
“The modern astrophysical concept that ascribes the sun’s energy to thermonuclear reactions deep in the solar interior is contradicted by nearly every observable aspect of the sun.” —Ralph E. Juergens