I have a few questions on filters.

Why go with 4x5.6 and not 4x4? My widest lens will be the new zeiss ze 15mm. (currently own 35,50,100)

Tiffen hot mirror or just irnd? What about scneider platinum ND? The prices ncrease rapidly..

Aby recommendations on pola filter? Keeping cost down is a priority. Can linear pola be used just as well on scarlet?

Ps! Can I shoot straight into the sun with heavy ND or do I need something more? Any experience on how much ND needs to be stack for this?