Query about a difficult problem. Do?

In a few individual IBS is very difficult to treat. If I consider that 4 doctors you have consulted have prescribed all type of treatment of IBS available including strict diet modification, you need to revisit the diagnosis. A lot of time, lack of evidence in small bowel evaluation,  Colonoscopy proves the IBS. But phase of disease may be responsible for faslely negative findings for other diagnoses like inflammatory bowel disease or non infectious colitis. Need to review the entire treatment you have taken and revisit everything from scratch. I myself had symptoms like IBS diarrhea variety which after 5 years is converted to diagnosis of Chron's disease.  Now I am better with specific treatment. 

Consult with every single medical records you have and with detailed history. You need to go for repeated investigations and only then you will feel alive again.