defiant 4 year old

My four year old is a very smart child and refuses to poo on the toilet. She will usually go outside and poo her pants take them off and just go about her ways. Other times she will start to poo her pants (do the majority) and then swan up to the toilet and do the last bit.
I know she is aware that she is pooing, when i ask her why she didnt go to the toile she will reply i just didnt make it. Her attention span is low, she refuses to look at me when we talk about it or any behavioral issue.
Latest development the other day i had told her she couldnt have an icy pole so she went outside stood in the yard and started to wet herself, my friend saw her and said "what are you doing" my daughter smiled and contiued to do a poo.
I have tried to ignore her not give attention which is hard with pooing, i have tried reward schemes and charts to no avail. I had her bowel X-rayed, she was constipated, doctors advise give her some laxitives.
PLEASE i am at my witts end, i am being called a bad parent dont know where to turn or what to do the problem has been occuring for over a year now.

[Kevin Kennedy, Ph.D.]

Ig a child has impacted bowels (more of a problem than 'simple' constipation), it can require weeks of treatment on a regimen of laxatives and stool softeners. I would ask her doctor about this. Beyond that, set a firm limit on her urinating or defecating outside, and discipline her with time out if it occurs.