The 2 down-right from your bad flag should have tipped you off; it is touching two bombs you had already flagged. That would show the one next to it (which you triggered) to be a bomb; the two under that one would clear the one left of it (above the 4), leaving just the four bombs around the 4. You could also have figured out the 4's surroundings just from it and the 2s right and down-left of it.
Graphically; blues are squares of consideration, red is conclusion.
\begin{array}{|c|c|c|} \hline &&&&&&&&2\\ \hline &&&\color{red}{3}&1&1&2&F&2\\ \hline &4&2&2&\color{blue}{2}&2&2&1&1\\ \hline 2&F&1&1&\color{blue}{F}&\color{blue}{F}&1 \\ \hline \end{array}
\begin{array}{|c|c|c|} \hline &&&&&&&&2\\ \hline &&\color{red}{F}&3&1&1&2&F&2\\ \hline &4&2&\color{blue}{2}&2&2&2&1&1\\ \hline 2&F&1&1&\color{blue}{F}&F&1 \\ \hline \end{array}
\begin{array}{|c|c|c|} \hline &&&&&&&&2\\ \hline &\color{red}{4}&\color{blue}{F}&3&1&1&2&F&2\\ \hline &4&\color{blue}{2}&2&2&2&2&1&1\\ \hline 2&\color{blue}{F}&1&1&F&F&1 \\ \hline \end{array}
\begin{array}{|c|c|c|} \hline &&&&&&&&2\\ \hline \color{red}{F}&4&\color{blue}{F}&3&1&1&2&F&2\\ \hline \color{red}{F}&\color{blue}{4}&2&2&2&2&2&1&1\\ \hline 2&\color{blue}{F}&1&1&F&F&1 \\ \hline \end{array}