Maximum tension in the string is in its lowest position.
`:.` Speed of mass `m` in its lowest position is
`v^(2) = 2gh = 2hl (1 - cos theta)`
`T_(max) - mg = (mv^(2))/(l)`
`T_(max) - mg + 2 mg (1 - cos theta)`
`= mg (3 - cos theta)`
Block of mass `4m` does not move.
So `mu (4 mg) ge T_(max)`
or `5 mu mg ge mg (3 - cos theta)or mu ge ((3 - cos theta_(0))/(4))`