(1) NO regulator needed?
IF you mean that the motor can run on any voltage from 3.5 to 6.4 then Vbattery of 3.6 to 4.2 is always in the range and you do not need a regulator. ie
- Vbat_min < Vmotor_Min < Vbat_max < Vmotor_max
(2) Buck Boost:
If you mean that you want to be able to set a motor voltage anywhere in the range 3.5V to 6.4V at any time regardless of battery voltage then you need a buck-boost converter. A boost only or buck only converter will not work in all cases.
If maximum efficiency is not crucial (and it may not be for 3 seconds of opoeration) Then the 48 cents MC34063 data sheet here will do your job. See fig 17 for one of several ways to achieve buck boost - efficiency can be better than what they say.
For better efficincy the eg TPS63060 is an example of an IC that uses syncronous rectification data sheet here About $4/1 at Digikey. This can provide over 1A out in buck or boost mode.
Example circuit only: