I have had damp proofing and new plastering done on a internal wall, there are salt patches and the plaster is still wet after four weeks

The damp proofing has not worked you should get the company back to rectify the problem

Have you been ventilating the room ie opening widows during the day and letting the moister out from the plastering is the room heated 4 weeks for the wall to dry would seem a long time I would contact the company that carried out the damp proofing to let them know and ask for there advice they should sort it out under guarantee if they are a reputable company. Kelvin If you like hit the like button