Where can I find the datasheet for an DGL-0401YG-4EH LCD display
From the picture you supplied it looks like a basic LCD (no drivers etc.) and one thing you can find is what the content of its capabilities are - try lightly pressing on the screen and get, in your minds eye, a picture of what the LCD can display. Hey, if you press too hard it'll bust but you can't make an omelette without breaking an egg or three.

Without knowing what it can display = no use to anyone so, try the pressure test and establish what it's capabilities are then, using a 1.5 volt battery on flying leads, go round each pin in turn and see if you can get any of the segments to flicker. More than likely there will be one (or possibly two) common pins that one battery terminal can connect to that "illuminates" several or all segments.

Establish this and you are nearly there - find a driver chip (plenty about) that interfaces with a "raw" LCD and that's just about job done. 

You don't really need a spec sheet - you just need a pin-out and a visual map of what the device can display. If you are interested in stuff like how it performs at 70 degC then this is more difficult to establish but only you can tell us that.