I cant really help with the dummy solution becasue my son lost his in our house somewhere one day and it had to be the last one i also had on hand DP and i turned the place upside down and for the life of me couldnt find it and still ahve not come across it and that was nearly a year ago. he slept well that night beleive it or not to my surprise that is!
but to him waking at 4 what time does he go down? does he still have two sleeps a day? if so maybe cut it back to one? or try holding out for another hour at night before you put him down. this is what i did and it worked wonders.
other mummies maybe more helpfull to you though!
I know what its like to have a newborn and a 2 yr at once bloody hard work last thing myou need is for him to be up at sparrow fart, prob just when you think you can crawl back into bed.LOL
all the best of luck to you!