The rasterized square (clear, therefore 4 as part of the font name) was the constructive basis. The intention was to put all characters within this grid and produce a highly structured, yet lively, resting in itself, display font. Relaxed but exciting, just.
An absolutely noteworthy detail are the classical construction principles (based on a typography book from the 50's for poster designers), the so-called optical weighting, derived and slightly exaggerated character elements: The characters are not purely symmetrical and the curve shapes do not close justified with the surrounding square.
Loops and tongues slightly hang over; the upper bows are slightly less protruding than lower ones, etc. The kerning is tuned to fit these design details: the white space between the characters match the same filling space.