DD has just had conjunctivitus and the only way I could put the ointment in her eyes was to wrap her in a towel to hold her arms down and then do it, but she's only 18 mths - I'm not sure if I could manage that with a 4 year old.
DS2 is 4 and just had a course of really disgusting antibiotics, we used bribery with him. Every time he had a spoon of medicine he got a chocolate to take away the taste. He needed it too, it was the foulest tasting stuff ever.
Good luck, sorry I couldn't think of anything more constructive!