Generating an equal number of random integers in response to a previous input in Matlab

I'm trying to create a random number matrix in matlab. However, I'm struggling a little with the logic of some of it. What I want is this:

I need it to loop through a predefined random matrix (2 rows, n columns) 50% of which are 1's and 50% are 0's (I can already do this part). Everytime it encounters a 1 it should then enter another loop which puts a 1,2,3 or 4 in the corresponding position in the second row. However (and this is the part I'm struggling with) I need it to have an equal number of 1's, 2's, 3's and 4's in the second row. So for example:

The matrix n = [0, 1, 0, 1, 1, 1, 0, 0; 0,0,0,0,0,0,0,0] should run through the script and produce something like: n = [0, 1, 0, 1, 1, 1, 0, 0; 1, 3, 2, 4, 4, 2, 1, 3]

This is what I have so far:

function pureToneTimer
ptpschedule = [0, 1, 0, 1, 1, 1, 0, 0; 0, 0, 0, 0, 0 , 0, 0, 0]
a = 0;
b = 0;
c = 0;
d = 0;
x = length(ptpschedule)/4
for n = 1:length(ptpschedule) 
    if ptpschedule(1,n) == 1
        while a < x && b < x && c < x && d < x
        i = randi(4)
        ptpschedule(2,n) = i;
        switch i
            case 1
                a = a + 1;
            case 2
                b = b + 1;
            case 3
                c = c + 1;
            case 4
                d = d + 1;
        end
        end
    end
end
assignin('caller', 'ptpschedule', ptpschedule)
end

Sorry if this turns out to be a really trivial question. I'm just struggling to wrap my head around it!

Thanks,

Martin