k1 = k1 /. Solve[Eqn4 == 0, k1][]
and then look at the value that k1 has.
/. is another way of writing
k1=ReplaceAll[k1,Solve[Eqn4 == 0, k1][]]
You can look up /. or ReplaceAll in the help system and see if this example helps make it more understandable.