No, PROC FMM does not support constraints on the mixing parameters. The RESTRICT statement is for the regression parameters only.
To control the mixing parameters, PROC FMM supports the PROBMODEL statement, but that statement does not support constraints on the mixing parameters.
BTW, if the ratio pi1 / pi2 = 2, then pi1 - 2*pi2 = 0, so this is just a linear restriction on the parameters.
I suppose it is possible to use PROC NLMIXED to maximize the likelihood of
Y = pi1*f1 + 2*pi1*f2 + pi3*f3 + pi4*f4,
which has only three mixing parameters. But unfortunately it will be much more effort than the PROC FMM code that you are trying to use.
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