You can split it into 4 sequences. then merge it back together to create your desired vector.
limit = 100;
a1 = 4:4:limit;
a2 = 5:4:limit;
b1 = 2:4:limit;
b2 = 3:4:limit;
a = sort([1 a1 a2]);
b = sort([b1 b2]);
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