Put in a wrist pin bearing and a wrist pin and measure the distance from the tin to the bottom of the wrist pin@ tdc and bdc. Subtract the bdc distance from the tdc and you should have a measurement of 76mm for a +4mm crank. You don't have to use a wrist pin, you could use the bottom or the top of the wrist pin hole on the rod if you wanted. Either way, it should get you close enough to tell whether it's a 4mill or not.