I didn't research PAM-4 enough to know whether each element is independent from prior values. The following code will generate a sequence of a specified length, randomly (and independently) selecting each element from the list of amplitudes. Maybe that is what you need, or you can adapt the idea.
% Set a random number generator seed
rng(2)
% Set the signal length and amplitude list
signalLength = 7;
amplitudeValues = [1, -.033, 0.33, 1];
% Generate a random index into the amplitude list
idx = randi(numel(amplitudeValues),signalLength,1);
% Get the amplitudes
signal = amplitudeValues(idx)
If you have the Statistics and Machine Learning Toolbox, you could use randsample to simplify the syntax:
signalLength = 7;
amplitudeValues = [1, -.033, 0.33, 1];
signal = randsample(amplitudeValues,signalLength,true)
