Alternate data in columns A and B (AAAB, AAAB repeat)

This might throw a row or two of errors, if the number of values in column A is not an exact multiple of 3. Try it out:

Formula:

=LET(

A,A2:B10,

B,ROUNDUP(1.33*ROWS(A),0),

C,1+MOD(SEQUENCE(B,,0),4),

D,INT(SEQUENCE(B,,0)/4),

IF(C=4,INDEX(INDEX(A,,2),1+D),INDEX(INDEX(A,,1),C+3*D)))