BP - it is not only the Glycol Ether but you may have also lost through evaoparation, some if not all ,of the Napthalene, Psuedocumene, and some of the NH4OH or ammonia. All are volatiles to a certain extent. According the the MSDS peroxides can form and when this happens product stability goes right out the window. Peroxides can degrade the action of what the product is intended to do.

I'd repalce the product and not buy in bulk.
Expecting 4 yrs of shelf life is a bit much in today's market. 16 to 18 months is the norm as I was employed in consumer products at one time and dealt with shelf life of the products I created.
JW