Well, it's wise to convert many formats to editing codecs, as even Premiere Pro can't deal with highly compressed 4K footage natively. 4K H.264 is extremely hard to handle....even if you build a $10,000 machine with 64GB of RAM and the top of the line GPU. Well, then it might be better. But still, for your system, it would be best to convert all to DNxHR (An Avid codec) or some other, editing codec. Cineform, if they have a 4K variant.