I’ve identified the four given windows as W1 (R234C234), W2 (R234C678), W3 (R678C234) and W4 (R678C678). As always with Windokus there are five hidden windows WR159C234, WR159C678, WR234C159, WR678C159 and WR159C159.
Prelims
a) R12C5 = {18/27/36/45}, no 9
c) R3C12 = {19/28/37/46}, no 5
d) R4C12 = {16/25/34}, no 7,8,9
e) R5C12 = {17/26/35}, no 4,8,9
f) R5C89 = {19/28/37/46}, no 5
g) R6C89 = {17/26/35}, no 4,8,9
h) R7C89 = {15/24}
i) R89C5 = {89}
j) 11(3) cage at R1C6 = {128/137/146/236/245}, no 9
k) 21(3) cage at R3C9 = {489/579/678}, no 1,2,3
l) 26(4) cage at R1C8 = {2789/3689/4589/4679/5678}, no 1
m) 26(4) cage at R6C6 = {2789/3689/4589/4679/5678}, no 1
n) 11(4) cage at R8C8 = {1235}
1a. Naked pair {89} in R89C5, locked for C5, clean-up: no 1 in R12C5
1b. 45 rule on R12 1 innie R2C7 = 4, placed for W2, clean-up: no 5 in R1C5
1c. 45 rule on C89 1 innie R3C8 = 8, placed for W2, R3C7 = 2 (cage sum), placed for W2 and D/, clean-up: no 2 in R5C9
1d. 45 rule on C12 1 innie R7C2 = 6, placed for W3, clean-up: no 4 in R3C1, no 1 in R4C1, no 2 in R5C1
1e. 45 rule on R89 1 innie R8C3 = 4, placed for W3, R7C3 = 3 (cage sum), placed for W3 and D/
1f. 11(3) cage R1C6 = {128/137/146/236} (cannot be {245} because 2,4 only in R1C6), no 5
1g. 2,4 of {128/146/236} only in R1C6 -> no 6,8 in R1C6
1h. 21(3) cage at R3C9 = {489/579/678}
1i. 8 of {489/678} only in R4C9 -> no 4,6 in R4C9
1j. Combined cages R89C89, R7C89 and R6C89 = {1235}{15}{26}/{1235}{24}{17}/{1235}{24}/{35}, caged X-Wing for 2, no other 2 in C89, clean-up: no 8 in R5C9
2a. 45 rule on R1234 1 outie R5C7 = 1 innie R4C5 + 3, no 1,7 in R4C5, no 1,3 in R5C7
2b. 45 rule on R6789 1 outie R5C3 = 1 innie R6C5 + 2, no 1,2 in R5C3, no 1,2 in R6C5
2c. 45 rule on C1234 1 innie R5C4 = 1 innie R7C5, no 3,6,8,9 in R5C4
2d. 45 rule on C6789 1 innie R5C6 = 1 innie R3C5 + 3, no 7 in R3C5, no 1,2,3,5 in R5C6
2e. 3 in R5 only in R5C12 = {35} or R5C89 = {37} -> R5C12 = {35}/[62] (cannot be {17}, locking-out cages), no 1,7
[Time to start using the hidden windows]
3a. 2 in R5 only in R5C24, locked for WR159C234
3b. 4 on D/ only in R1C9 + R5C5 + R9C1, locked for WR159C159, clean-up: no 5 in R2C5, no 6 in R5C8
3c. R34567C5 = {145}{27}/{145}{36}
3d. R3C5 + R5C6 (step 2d) = [14/47/58/69] (cannot be [36] which clashes with all 6s in C5), no 3 in R3C5, no 6 in R5C6
3e. R34567C5 = {145}{27} gives 2,7 both in 22(5) cage at R4C5 (because R5C4 = R7C5, step 2c) and no 3,6 in 22(5) cage
3f. R34567C5 = {145}{36} gives 3 in R46C5 and no 2,7 in 22(5) cage (because R5C4 = R7C5, step 2c)
3g. 22(5) cage must therefore contain both of 2,7 but not 3,6 or 3 but not 2,7 = {12478/13459/13468} (cannot be {12379/12469/12568/13567/23458/23467}, 1 locked for R5, clean-up: no 9 in R5C89
3h. 8,9 only in R5C6 -> R5C6 = {89}, R3C5 = {56} (step 2d)
3i. Killer pair 3,6 in R5C12 and R5C89, locked for R5, clean-up: no 3 in R4C5 (step 2a), no 4 in R6C5 (step 2b)
3j. 7 of {12478} only in R6C5 -> no 7 in R5C45, clean-up: no 7 in R7C5 (step 2c)
3k. 3,5 of {13459} must be in R6C5 and R4C5 (cannot be 4{15}93 which clashes with R5C3 + R6C5 = [53], step 2b) -> no 5 in R5C45 + R6C5, clean-up: no 7 in R5C3 (step 2b), no 5 in R7C5 (step 2c)
3l. 3 of {13468} must be in R6C5 -> no 6 in R6C5, clean-up: no 8 in R5C3 (step 2b)
3m. 1 in R5 only in R5C45, CPE no 1 in R46C4 using diagonals
3n. 8 in R6 only in R5C67, locked for hidden window WR159C678
3o. 5 in C5 only in R34C5, locked for hidden window WR234C159, clean-up: no 2 in R4C2
3p. 1 in C89 only in combined cages R89C89, R7C89 and R6C89 (step 1j) = {1235}{15}{26}/{1235}{24}{17}, no 3,5 in R6C89
4a. 27(5) cage at R3C5 = {13689/15678} -> R5C7 = 8, R5C6 = 9, placed for hidden window WR159C678, R4C5 = 5 (step 2a), R3C5 = 6, placed for hidden window WR234C159, R5C3 = 5, placed for hidden window WR159C234, R6C5 = 3 (step 2b), placed for hidden window R678C159, clean-up: no 4 in R3C2, no 2 in R4C1, no 1 in R4C2, no 3 in R5C12
4b. R5C12 = [62], 6 placed for hidden window WR159C159, clean-up: no 4 in R5C8, no 2 in R7C5 (step 2c)
4c. Naked pair {34} in R4C12, locked for R4
4d. 3,5 of 27(5) cage only in R3C6 -> R3C6 = {35}
4e. 27(5) cage = {13689/15678}, 1 locked for R4 and W2
4f. 3 in W2 only in R23C6, locked for C6
4g. Caged X-Wing for 3 in R5C89 and 11(4) cage at R8C8, no other 3 in C89
4h. 3 in C9 only in R59C9, locked for hidden window WR159C159
4i. 25(5) cage at R5C3 = {12589/14578}
4j. 25(5) cage = [52]{89}1/[51]{78}4 (R7C45 cannot be [21/14] which clash with R7C89) -> R6C3 = {12}, R67C4 = {78/89}, 8 locked for C4 and W3
4k. Killer pair 1,2 in R6C3 and R6C89, locked for R6
4l. Killer pair 1,4 in R7C4 and R7C89, locked for R7
5a. 21(3) cage at R3C9 = [498/768] -> R4C9 = 8, placed for hidden window WR234C159
5b. R2C8 = 6 (hidden single on D/), R4C8 = 9, R3C9 = 4 (cage sum), placed for hidden window WR234C159, R4C12 = [34], clean-up: no 7 in R3C2, no 2 in R6C9, no 2 in R7C8
5c. Naked pair {17} in R4C67, 7 locked for R4 and W2 -> R23C6 = [35]
5d. 26(4) cage at R1C8 = {4679} (only remaining combination) -> R1C8 = 4, placed for hidden window WR159C678, R12C9 = {79}, 7 locked for C9, R5C89 = [73], 7 placed for hidden window WR159C678, clean-up: no 1 in R6C89, no 2 in R7C9
5e. R6C89 = [26], 2 placed for W4, naked pair {15} in R7C89, locked for R7, R7C5 = 4, R5C45 = [41], 1 placed for both diagonals, R4C67 = [71], 7 placed for D/, R1C9 = 9, placed for D/
5f. R1C7 = 6, R2C6 = 3 -> R1C6 = 2 (cage sum), 6 placed for hidden window WR159C678
5g. R67C6 = [48] = 12 -> R67C7 = 14 = [59], 9 placed for D\, R67C4 = [87], 8 placed for D/, 7 placed for W3
5h. R9C678 = [135], R9C9 = 2, placed for D\
5i. R3C3 = 7, placed for D\ and W1, R4C34 = [26], 2 placed for W1, R3C4 = 3 (cage sum)
and the rest is naked singles, not using the diagonals or windows.