Only the albumen should give a purple colour.
2 The students should not be prepared to make a generalization about all proteins from a single reaction with albumen solution. If albumen is a typical protein, then it is proteins that will normally give the biuret reaction. In fact it is given by all substances having at least two peptide linkages (-CO-NH-) which includes all proteins and most peptides.
3 It could be argued that sodium hydroxide and copper sulphate will produce the characteristic purple colour whether or not proteins are present but the reaction is suppressed in the presence of glucose or starch. The control with test-tube 4 makes this explanation less likely.