asol = solve(a,'returnconditions',true)
It looks like x >= 4 is the solution.
When you have a problem with multiple solutions, certainly an inequality like this, you need to tell MATLAB to describe the solution properly. When all I do is use solve on a, what does it do? It gives me AN answer, a valid solution to the problem.
Read the help docs for solve, where it explains the use of this property.