Anshul,
It very strongly NOT recommended to have mismatch between client tools and Platform. The problem you will run into is if some updates / creates a WebI BI 4.0 w/ SP6 doc using BILauch Pad and then opens that same doc with WebI BI 4.0 w/ SP4 rich-client they could corrupt the file.
The only way you could still use the BI 4.0 w/ SP4 client tools is to prevent Everyone from updating / creating anything NEW via BILauch Pad. This way you can control what tool / version is updating and you have better control of you mismatch.
Regards,
Ajay