In order to calculate R_th, you have to made all voltage and current sources equal to 0 (creating open circuits at current sources and shorts at voltage sources), so you will wind up with only resistors. That means that:
$$R_{th}=[(R_1+R_2)^{-1}+1/R_3+1/R_4]^{-1}$$
The dependent voltage source has a transresistance, so it adds on to the value of R_th. This is not being accepted as my answer though. I tried with and without the transresistance, even though it shouldn't be removed.
For \$\small V_{TH}\$, let \$\small V\$ be the 25/55/25v node, then: \$\frac{V-4I_x}{29}+\frac{V}{55}+\frac{V-25}{20}=0\$, and \$I_x=\frac{V}{55}\$. Hence \$\small V_{TH}=V-25\$.
When in doubt try to go back to the definitions:
simulate this circuit – Schematic created using CircuitLab
This means that V_th is just the same as the open-circuit voltage across the load. Similarly you can show from the definition that I_n is the short-circuit current across the load. The equivalent resistance is then V_th / I_N.
