In that case why not use the
SAP BI 4 Resource Usage Estimator (aka Sizing Estimator) in the same link provided earlier. It would help you to get the SAPS for your existing environment.
The requirements 4.0 and 4.1 are same except for few versions of JVM etc for which you can get the information from the PAM for BI4.1. https://service.sap.com/PAR
Are you thinking of upgrading the software in same server box? I would not suggest to use the same box unless it is a test / dev environment. Even for that you need to ensure that the drives are different.
For 4.x the minimum requirement is 16 Gb for installation and for normal usage it would be ideal to have atleast 32 Gb(With reference to PAM 4.x)
Get a better idea for upgrade http://scn.sap.com/docs/DOC-9884
You would get very good ideas of preparing for a general upgrade activity @ http://wiki.scn.sap.com/wiki/display/BOBJ/How+to+Upgrade+to+BI4.0