My apology if someone already mentioned this and I missed it ... but ... My 5 inch locks up with a ball detent in the frame ... and the 4 inch locks up on the ejector rod tip.
My 5 inch uses a DX/Classic type front sight (slides into a spring loaded shoe) ... and the 4 inch uses a pin on sight.
If you are only going to be doing steel challenge (with an optic) then neither will be a worry ... you won't be doing (on clock) reloads or snap in front sight changes.