I have a gtx780 gigabyte with 1 ddr5 k4g20325fd-fc03. can it be replaced with k4g41325fd-fc03?
My card has 3gb and density is 2gb for first code and 4gb for second. Thanks

In theory - yes, but only if the same as your type graphics card was manufactured and had 6Gb memory; and you can take bios dump from that card.
This is a minimum requirement, but maybe will need change some configurations straps on board. For understand need look on both models and compare straps positions.

if you replace 1 ic from all - no, only on k4g2
if you replace all to k4g4, than flash bios, may be change straps on board