4V is a good reading. The spec at idle is 5V +/- 1V, but at 1200 rpm, it should be higher because the relay should be trying to increase the voltage to bring the idle down.

4V should control the idle. I would look for air or vacuum leaks.

__________________
Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe