I love cobs they’re great but for a 2x4 ur looking for a led strip light , or look at optic phatslim 1xl a mix of strip and cobs 2 of those would kill a 2x4
A 4 inch fan should be more than enough for a 4x4 that’s 200cfm being moved that’s ur whole tent twice a minute so like somebody said unless ur running open hood hps or growing in like satans attic u should be fine
You should only need a 4 inch if that’s the only tent u are using it for, and for the filter the ac infinity one is top of the line or Check this link out these have more surface area open for less drag due to hexagonal holes and not just holes...