hi

"no significant herniation" is great for you.
... although MRIs are not 100 percent, an MRI can miss a finding due to "slicing" of images throughout your spine. They're great, but not perfect...not that your MRI missed anything in your case...

"small 4mm cyst - uncertain clinical significance..." okay. cyst findings are not that popular on spine MRIs.

most cysts in general are benign; non-cancerous.

could be a congential anomaly - from birth and of no medical concern

a biopsy would tell you something but probably not worth the risk of injury or harm that a biopsy procedure could cause at that level.

could retake an MRI, to see if cyst grows, but not likely without a correlation of symptoms to a cyst at that level.

"s3" I take it to be your 3rd sacral vertebra = middle of tailbone

hip and leg length discrepancy may be separate issues - maybe ortho consult in your future

hope that helps.