1st pack of lifepo4 16s20p for powerwall

My plan is to build 4 sets in parallel of 16s20p of lifepo4 for powerwall...

Normal load: 20A - ~ 30A
Peak: 50A

So that should be from 1A - 2.5A per cell for a single pack and would definitely lower in 4sets.

Q: is it ideal to use 4A cell level fuse? May it be wire/fast blow glass type?

Not sure what your space requirements or restrictions are but maybe build 20p packs and put them in series. And then you can add 20p packs so that in the end you have one battery that is 16s80p but the 80p is broken up into 20p packs that are parallel. Otherwise you are looking at a lot of BMS wiring.

Thats the final plan 16s80p(20p per pack) of 16string but for the initial phase would be 16s20p for now... just not sure for the cell level fuse of 4A...

1st phase:
|-20p-20p-20p-20p-20p-20p-20p-20p(-)
|-20p-20p-20p-20p-20p-20p-20p-20p(+)

Then add 20p and so on till 80p.

Cell level fuses are not to prevent of over load protection to the system. It is provide safety to a cell that has self shorted and protect it from the rest of the parallel cells. 4A would be sufficient. You could even go with 10A and still be fine. You have 20p, and each cell has the potential of delivering at least 2A at half charge. That's at least 60A of current that could possibly flow in to a shorted cell.
For more details, please read the FAQ about fuses