Could the decrease you're looking for be slip one, knit two together, pass slipped stitch over the two you just knit together? This will be a double decrease. In order for your square to stay square, you just have corresponding increases to offset the decreases.
If the square had eyelets along the edges, you could do a yarn over increase inside each edge and do the double decrease in the middle and they would balance out.
"Trust the pattern!"
Time flies like an arrow, but fruit flies like a banana.
Hop aboard while we follow the cloud-covered van of knitting harmony, man!! Woooo!! --Hedgehog