hi guys i m driving a 300se w 140 which come wit auto 5 spd in malaysia.i got problem wit my gear selector as it doesn stick to D but stick only at 4.so after using sometime i got to fix this up.

my mech advise to change to the auto 4 spd from a s280.this is much cheaper to mine to rebuilt or replace.n he said will give less problem n cheper to fix in future.

can someone give me advise n any opinion on this pls

Yes, what your mechanics said was true. The parts require to fix the 5th speed alone cost more than the rest of the transmission rebuilt. However, replace only the 4 speed transmission alone isn't enough, several things you will/might have to replace are

1. Rear differential
2. Rear axle shafts
3. 4 speed trans. is shorter therefore you will need a new propeller shaft
4. If you don't manually shift the transmission then you might still keep the old transmission seclector
5. ECU <------- I'm not sure about this

Not entirely clear what he swapping (722.5 for a .3/.4?) but I would say if the mechanic thinks he can do it let him try. Clearly the 4 speeds are cheaper to rebuild and to purchase as rebuilt units. I would say most people consider them more durable as well.

how bout the performnace guys?n the fuel consumption?will it be bad?

Fuel economy will likely suffer a little but it shouldn't be that bad - maybe a mile or two per gallon. At the link below you can track some Mercedes that transitioned from 4 speeds to 5 speeds in the 1995 to 1997 timeframe.

http://www.fueleconomy.gov/feg/findacar.htm