The probable source of your error is the first two conditions in the if block - they result in E_n being a 4x1 vector since e is a 4x1 vector. Only the last condition in the if block will result in E_n being a scaler.
...
while j <= 4
while i <= 7
if M(i,j) > -pi && M(i,j) < 0 || M(i,j) > pi
E_n = M(i,j) - e; % <-------------------------------------------- 4x1
elseif M(i,j) < pi && M(i,j) > 0
E_n = M(i,j) + e; % <-------------------------------------------- 4x1
elseif M(i,j) == 0 || M(i,j) == 180
E_n = M(i,j); % Scaler
end
while error >= 10^-6
E_n_plus_1 = E_n + ((M(i,j) - E_n + e(j) * sin(E_n)) / (1 - e(j) * cos(E_n)));
E_n = E_n_plus_1;
error = E_n / E_n_plus_1;
end
E(i,j) = E_n_plus_1;
i = i + 1;
end
j = j + 1;
end
