To be honest I don't know the exact answer, but I think I have some info to offer. AnyConnect premium support IPSEC and AnyConnect Plus supports SSL. Are you using IPSEC for the premium license or SSL?
If using SSL, the Plus licensed device should be able to support both versions. I thought with the premium license you could use AnyConnect 4.1 and 3.1 on the same device. Not sure if your license agreement legally supports this. With the new Apex license to use 4.1, when you enter the license, even for 1 user, it maxes out the premium users on your ASA. Its on the honor system.
I don't use the Plus license. License on my home device:
AnyConnect Premium Peers : 25 perpetual
AnyConnect Essentials : Disabled perpetual
Other VPN Peers : 25 perpetual
Total VPN Peers : 25 perpetual
Essentials is disabled because I don't use SSL. I am sure others will have additional information to add. You may just want to call your Cisco rep.