hi M.. i just had to let you know my oldest son's experiences with my grandson. i am not really sure if all is the same with you, but thought it might give you another route to think about.
my grandson is also 4. potty trained pretty well with the potty part, but to go #2, they tried for months and months and months. took everyone's advice, tried it all, from all that u have done with your daughter to whatever else they could come up with. my grandson would hold it for days and days, and then scream or cry when he did go, and would most of the time at least start to go in his pants.
christopher, the dad, is kinda a health food nut. they had already tried limiting taevin's dairy(can cause constipation), so one day out of the blue, christopher decided to try soy milk(vanilla flavored) with taevin cause that boy likes his milk! LOL. he liked it, and christopher said they were shocked with what happened within a day of taevin having the soy milk. he said that child started going #2, and he said initially what started coming out of him shocked the daylights clean out of them. they said the size of what came out, and the amount was overwhelming.(sorry to be exlpicit) christopher said he as a grown man, couldn't have imagined what came out of taevin. he said he went like that several times over a couple days, and christopher said he has no idea where it all was at in his little body. christopher said it has been like a miracle though, cause taevin is completely regulated now, goes in the toilet with absolutely NO problem every day, and is a very proud little boy for doing so!
so just a thought. ya might want to try it. the way i look at it, it might be at least, worth the try to see.
good luck, and take care!
C.