@ Per

Your sequence for the U 4-flip is optimal in ftm but also in qtm.

Here is the complete list of all 16 solutions that are optimal in both metrics.

Note, that these algorithms do not rotate any of the centers:

F U2 F' U L' U' B' R' U2 R U' B U L (14 ltm, 14* ftm, 16* qtm)

F U R U' B U2 B' R' U' F' U L' U2 L (14 ltm, 14* ftm, 16* qtm)

F' U2 F U' R U B L U2 L' U B' U' R' (14 ltm, 14* ftm, 16* qtm)

F' U' L' U B' U2 B L U F U' R U2 R' (14 ltm, 14* ftm, 16* qtm)

R U2 R' U F' U' L' B' U2 B U' L U F (14 ltm, 14* ftm, 16* qtm)

R U B U' L U2 L' B' U' R' U F' U2 F (14 ltm, 14* ftm, 16* qtm)

R' U2 R U' B U L F U2 F' U L' U' B' (14 ltm, 14* ftm, 16* qtm)

R' U' F' U L' U2 L F U R U' B U2 B' (14 ltm, 14* ftm, 16* qtm)

B U2 B' U R' U' F' L' U2 L U' F U R (14 ltm, 14* ftm, 16* qtm)

B U L U' F U2 F' L' U' B' U R' U2 R (14 ltm, 14* ftm, 16* qtm)

B' U2 B U' L U F R U2 R' U F' U' L' (14 ltm, 14* ftm, 16* qtm)

B' U' R' U F' U2 F R U B U' L U2 L' (14 ltm, 14* ftm, 16* qtm)

L U2 L' U B' U' R' F' U2 F U' R U B (14 ltm, 14* ftm, 16* qtm)

L U F U' R U2 R' F' U' L' U B' U2 B (14 ltm, 14* ftm, 16* qtm)

L' U2 L U' F U R B U2 B' U R' U' F' (14 ltm, 14* ftm, 16* qtm)

L' U' B' U R' U2 R B U L U' F U2 F' (14 ltm, 14* ftm, 16* qtm)

An optimal solution in Layer Turn Metric (ltm) that does not disturb the centers counts 13 moves.

So, this algorithm can beat yours at least in ltm:

CR2 MR2 D' MF2 U R2 MF L2 U MF2 D' R2 MF' R2 (13 ltm, 18 ftm, 28 qtm)

SR2 D' MF2 U R2 MF L2 U MF2 D' R2 MF' R2 (13 ltm, 18 ftm, 28 qtm)

If we don't care about centers, then an optimal solution in ltm would require only 10 moves:

MF2 L2 MF L2 U MF2 R2 MF' R2 U' (10* ltm, 14* ftm, 22 qtm)