Okay, maybe it's me, but I found a better interpretation for the last part of 4c with Athefre's method.
Technically, when you finished 4b and put the DF/DB edges diagonal, the center shows you the direction of the M move. Simply move it to the direction of the matching edge (matching with UB -> M', matching with UF -> M), then U2, and the rest is pretty obvious.
A few examples:
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Pretty normal:
Setup:
D2 L2 R2 U L' R F D U' L D' U F' x'
Solve:
U' M U M' U M U2 // now it's time to predict the DF/DB edges: after M2, the the one in UB goes to DF
M2 U // diagonal DF/DB. Now the center matches with the UB sticker, so next move is M' U2
M' U2 M U2 M2 // finish
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Pretty normal:
Setup:
U R2 U2 R' U B L U' F L' B' R'
Solve:
M' U M U' M' U2 M' U' // now it's time to predict the DF/DB edges: after M2, the the one in UB goes to DF
M2 U // diagonal DF/DB. Center again matches with the UB sticker, so M' U2 again
M' U2 M' U2 M2 U2 // finish
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Pretty normal:
Setup:
D2 L2 F2 R2 U R2 F2 L' R' B' L2 R2 x
Solve:
M2 U' M' U2 // now it's time to predict the DF/DB edges: after M2, the the one in UB goes to DF
M2 U' // diagonal DF/DB. Center matches with the UF sticker, so it's M U2
M U2 M U2 M2 // finish
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Tricky recognition:
Setup:
R2 B2 L2 U R2 F2 R2 U L R' x
Solve:
M U' M' // tricky one: see the sticker on FD? It's a DF/DB sticker going to DB
U2 M U // diagonal DF/DB. Center matches with the UF sticker, so this time it's M U2
M U2 M // finish
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Tricky recognition:
Setup:
B L' F U2 D2 B' R F' D' F2 B2 U R2 L2
Solve:
M' U M' U M U // tricky: see the FU sticker? It's a DF/DB sticker going to DF
M U' // // diagonal DF/DB. Center matches with the UF sticker, so it's M U2
M U2 M U2 M2 U2 // finish
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An interesting one with misoriented centers:
Setup:
B F U2 L' U2 B2 U2 L' B F' L2
Solve:
M' U M' U2 M U2 // now it's time to predict the DF/DB edges. Let's say our DF/DB edges are currently on UL and UF. Interestingly, it is still working if we chose the other two edges (currently on UB and UR) as our DF/DB edges! It leads to the same solution.
M2 U' // diagonal DF/DB. Center matches with the UF sticker, so it's M U2
M U2 M U2 M // finish
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Again with misoriented centers:
Setup:
B2 L R' U2 B' L2 R2 F2 L R' D' L R' x'
Solve:
M U M U2 // now it's time to predict the DF/DB edges: we can chose between UB+UL, or UR+UF (it's the same)
M2 U // // diagonal DF/DB. Center matches with the UB sticker, so it's M' U2
M' U2 M U2 M // finish
