Hi sir;
how can i model reducing elbow of 4 x 3 in caeser..

A reducing elbow is not a "Code Defined Fitting", and as such CAESAR II doesn't have this item. Most user who need this item break the elbow into two pieces, and arbitrarily change the diameter on the 2nd half. You're on your own as far as SIFs go.

Further to Rich's response...
Assuming a 90 degree elbow - model this as two 45 degree elbows, one will be 4in and the other 3in. It will take a little math. Draw a tangent line at the 45 degree point around the elbow. There will be three nodes on this tangent - a node at each end where this line intersects the pipe centerlines entering and leaving the elbow and that tangent point. Call out a bend at one end and the other of that tangent line and change pipe size at its midpoint.
I guess you could get fancier if your 4x3 geometry is not symmetric about the center.

can you please provide Any caesar model for reduced elbow modelling.