What techniques can determine the concentration of ethyl acetate in a solution containing ethyl acetate, acetic acid, ethanol and water?
You might consider taking Aliquots of the reaction (or if you have the equipment directly sampling via a cannula) into a GC-FID or quantitative GC-MS. This way you could monitor the concentrations of all species in solution given the right GC method. Areas under each peak will give you relative amounts, you would need to use an appropriate known amount of an internal standard that remains constant to get absolute numbers.