Rochr, your example of a 4200x6000 image being used for a 70cm by 100cm poster works out at approx 150 dpi, not 72dpi, which is I believe, fairly usual for poster size, and so should be acceptable to look at. If you printed that image at 72dpi, it would end up around 211cm high. 

At the end of the day, (for example) a 4000x3000 at 72dpi image has the same number of pixels as a 4000x3000 image at 300dpi, the only difference is the output size of the print, as the dpi (which really should be ppi) is telling the printer to fit so many pixels into a certain area.

There is another option, and that is plugins (ie Genuine Fractals, or SizeFixer) for Photoshop that can enlarge pictures with minimal quality loss, so doing a smaller render size and upsizing could be quicker too. Not cheap, but affordable, especially if you need the time.

http://www.fixerlabs.com/New_Website/pages/sizefixer.htm

http://www.ononesoftware.com/detail.php?prodLine_id=2

Another option I suppose is do you know anyone who has a few computers you could use for a Bryce Lightning network render?