Solve the circuit using Thevenin
Douglas, the question regarding the voltage across \$R_4\$ should be easily solved using your own approach. CASE 1 below shows what I believe you arrived at, though as I said I think you miscalculated the resulting \$R_{TH}\$. I've shown what I think the equivalent should be on the right side of it. You should be able to easily apply Ohm's law now to work out the voltage across \$R_4\$. Compute the total current and apply that to \$R_4\$.







simulate this circuit &ndash; Schematic created using CircuitLab

I've also shown above your \$R_6\$ question. I hope I followed the approach you were being taught by breaking the connections around that resistor. From there, it's very simple to work out the value of \$R_{\vert\vert}\$, replacing four resistors with just one. The rest should be again quite easy to do, as you have a simple circuit and it should be easy to calculate the total current in it. That will also, of course, be the current in \$R_6\$.