glow plug light on c 220d w202

hi my car has just had glow plugs renewed only done 3 as one at bulk head ok
start up then glow plug light stays on for around 40 seconds.
had relay checked ok one plug not working other two ok any help would be great should i renew 4th one at bulkhead'
car is 1998 r reg c class c220d at genuine 75k

 

Can't hurt..

 

I would renew the fourth one.

 

hi i intend to change no 4 plug tommorow will update when done thanks sam