Quadrant standard benchmark app from the market. Have a look at system/hardware info. section, CPU speed:
S4 = 1.5GHz
S3 = 1.7GHz
Yup, if you tell them about the CPU etc. they should make sure that you get to keep the S4 version and if you do get the S3 back then you have every right to get a full refund or to get another new model with the S4 due to said reasons.
If there is only a problem with the screen though, then I think they will just replace the screen rather than give you a brand new one, generally that is what happens with refurbishments, they would replace the chassis, battery and screen and leave everything else as it is (well I think that is the case with "3" anyway)