1. I have to make 5V from single Li-ion cell, whose voltage can vary from 2.7V to 4.2V.
My current need at 5V is 400mA (max)

2. I have selected TI's TPS81256 for this application. This is meant for my application.
Just one output capacitor required, & thats it.

3. TPS81256 works at 4Mhz. So it generate noise & it can effect my circuit as it has lots of analog cicuit.

4. What value of decoupling capacitor I should use to filter this high frequency noise?

5. Xc = 1/2*pi*f*c
So I need Xc to be infinite for low freq & zero for high freq (ideally for bypass caps)
So I think using 4.7uF, 10uF & 0.1uF in parallel will be sufficient?

 

Whether it's sufficient or not depends upon the circuit layout and grounding (do you have a ground plane?), and the sensitivity of your circuit.
If that's not adequate you may need to add an inductor in front of the decoupling caps to provide more filtering.