Perfect 4 square all along the length of the stock isn’t a reasonable expectations. Quite a lot of flattening is to be expected after the lamination is done.

Going slowly and adding one board at a time with special attention given to good alignment of the top surfaces would be my main recommendation. There should not be any gaps appearing below the surface when flattening if you use enough thinned glue. Trying to laminate 4 long boards is a bit overwhelming the first time. Adding one at a time is easier.