If using a for loop is not actually a requirement you can do this very simply as such:
myMatrix = rand(5,4); % create 5x4 matrix
myMatrix(3,:) = 1:4; % replace values
If the for loop is a requirement:
myMatrix = rand(5,4); % create 5x4 matrix
for n = 1:4
myMatrix(3,n) = n;
end