Thévenin equivalent circuit
For RTH, you have 4 + (520 V bypasses 260 ==>0) + 40 (current source in series with R is identical to a current source ==>0) + 6 = 50 ohm.

For VTH, measure the open circuit voltage. The 1 A flows through the 40 ohm, generating -40 V. The 520 adds, giving 520-40 = 480 V.