Community Tip - Your Friends List is a way to easily have access to the community members that you interact with the most! X
Solving directly as Alan had shown sure is the better way, I guess.
If you still want to use the solve block, the guess for X has to be a 4x4 matrix.
The symbolic solution is very cumbersome and unhandy. Mathcads symbolics still has big problems with vector and matrix equations and often will solve for single elements only.