You need to find the payload for it, if it has been made and install it the same you did the first. Not sure sure if the Hermes payload has been made by PSFreedom as KaKaRoTo isn't a big fan of his work as he feels its not coded properly but he has created the equivalent but better coded which is PL3.
Someone might have done the Hermes v4b for N900 i'm not really sure. If you already have it then install it the same you did with last one.
i got email from drizzt84 and he said he willing to compile the psfreedom.ko hermes v4 for N900 (not the whole gui) just psfreedom.ko which is what we need. Still waiting on it.