I need some help “splitting one column”

hi… i’m working on Studieren-Info theme [i’m a n00b]
this is the result by now https://www.afiua.com/rovato … actually i’m using colors only to show different areas… i still have to work on that… i’d like to finish the structure before…

basically i’ld like to split the colunm with the posts after the first post … so i’ll have 4 col instead of 3…

I did some “experiments” but actually i didn’t get how to do it ??

[Title modified to be a little more descriptive – spencerp]