Tentative idea for 4x4 direct solving method
My attempt to do something similar to stadler, but hopefully not as inefficient.
(Note: the notation I use has l' r = M', not l' r = x)
1. FB (1x3x4 on L)
2. SB (1x3x4 on R) with <l, r, R, U>
3. CMLL
4. L28P = L12E + L4C with <l, r, U>
Obviously the only really new part here is the last step, so here's my approach:
4.1. Orientation
4.1a. OBlock: orient a 4x3x1 on l slice (so U/D colors only on D, F/B colors only on the l half of F/B centers)
4.1b. EO with <r, U> (one r/r' flips all edges on r, and
this is analogous to 3x3 arrow case)
4.1c. OL4C: orient the last 10 center pieces (w/ <l, r, U> Niklas)
4.2. ULUR with <l2, r2, U> + M' U2 M
4.3. Permutation with <l, r, U2>
4.3a. Pair L8E
4.3b. L4C (3x3 redux)
4.3c. L4E (+PLL parity)
Pros:
No OLL parity
Rotationless
Lookahead in step 4 easier than in cage/stadler (EO can be detected w/o looking at B/D, reduction-based approach -> don't have to look for specific pieces -> cases are easy to come by, always solving a specific thing involving very few pieces -> things are easy to track and identify, similar to 3x3 LSE)
Centers don't take forever like in cage/stadler
Direct solving is fun, slice turns are fun
Cons:
Lookahead may be hard in F2B
Without being able to plan whole FB in inspection, effieciency is non-ideal
OL4C, L8E, and L2C currently still take more moves than I'd like
No easy way use 3x3 hax like CMLLEO or ULUR influencing during EO (though I was thinking one might be able to influence OL4C during EO)
Still developing algs, but current average movecount estimate based on own solves:
50 F2B + 10 CMLL + (8 Oblock + 6 EO + 16 OL4C) + 8 ULUR + (16 L8E + 16 L4C) + 5 L4E (+ 6 PLL parity) = 135-141 STM
Compare with me going to
cubesolv.es, opening up a bunch of ~30second 4x4 reconstructions, and getting an avg Yau/Hoya movecount of (131 + 120 + 154 + 139 + 137 + 124 + 134 + 173 + 176 + 158 + 162 + 158 + 161 + 155 + 135 + 136 + 137)/17 = 146.47 STM