How do you draw a tetrapeptide from the one letter codes(Amino acids) and also give the pI, if given the pKa values?

Question

For example the Amino acids: S,T,G,L

So i know i decipher the the codes then to make tetrapeptide i start with the n terminus to the c terminus by adding the r groups? How do i figure out the pi? just take the average of the pka values? Any help would be helpful.