You are right.

The exam code for an EMCIEe for SAN Implementation Engineer credential is E20-855.

This URL I copied from the EMC Education Services portal should take you to the exam description. 
https://education.emc.com/content/_common/docs/exam_descriptions/e20-855_IE_SAN_Implementation_Expert_Exam.pdf

I checked and see that either one of these credentials qualify as the specialist level pre-req for a NAS expert credential. 

Implementation Engineer, Network Attached Storage (NAS) Specialist Version 5.0 (EMCIE)  E20-361 exam or

Implementation Engineer, Networked Storage - SAN Specialist Version 4.0 (EMCIE) E20-351 exam

If you are also planning to pursue the EMCIEe for NAS Implementation Engineer credential I checked on specialist level pre-reqs.

Either version 4.0 or 5.0 of Implementation Engineer, Network Attached Storage (NAS) Specialist
credentials are acceptable pre-reqs.

Since you don't state the exact credential name you hold, I urge you to send an email to EMC Proven Helpdesk@emc.com and get confirmation
that the credential you have applies to the NAS expert pre-requirements. 
They can also have some specific advice on how best to meet your goals.

Good luck

Thank you.

Only one question left. Is there a pre requisite specialist exam before being able to take the E20-855? If yes. what is the exam code for it?

Judging from the answers given, can I use my NS Specialist 4.0 qualification for both the EMCIEe for SAN and EMCIEe for NAS?

Thanks

The ES20-855 is the only exam required to earn the IE SAN expert credential.

Your IE Network Storage Specialist ver 4.0 credential is only good for the NAS expert credential.

There is another similarly named credential IE Networked Storage SAN Specialist 4.0 that is good for the SAN expert.  Unfortunately your IE Network Storage Specialist 4.0 is not an acceptible pre-req to the IE SAN expert credential.

Thanks.