According to the International Bureau of Weights and Measures (Section 5.5.3) a value should be formatted with a space between the number and the unit (except for degree, minute and second of a plane angle).
While it’s quite practical that SketchUp allows for writing “4mm” instead of “4 mm” (one less keystroke) I think SketchUp should still support the correct way to enter the value. Currently SketchUp refuses to parse “4 mm” and wrongly calls it an invalid length.