I think you can upgrade only two switches to EMI as per the attached document

Master Switch Election
The stack behaves as a single switching unit that is managed by a master switch elected from one of the member
switches. The master switch automatically creates and updates all the switching and optional routing tables. Any
member of the stack can become the master switch. Upon installation, or reboot of the entire stack, an election
process occurs among the switches in the stack. There is a hierarchy of selection criteria for the election.
1. User priority—The network manager can select a switch to be master.
2. Hardware and software priority—This will default to the unit with the most extensive feature set. Catalyst 3750
switches with Enhanced Multilayer Software Image (EMI) versions take precedence over Standard Multilayer
Software Image (SMI) versions.
3. Default configuration—If a switch has pre-existing configuration information, it will take precedence over
switches that have not been configured.
4. Uptime—The switch that has been running the longest is selected.
5. MAC address—Each switch reports its MAC address to all its neighbors for comparison. The switch with the
lowest MAC address is selected.

also

Cisco IOS Images Must Be Identical
The Cisco StackWise technology requires that all units in the stack run the same release of Cisco IOS Software. When
the stack is first built, though, the units can have SMI and EMI version differences. This will automatically cause the
EMI versions to become the master switches so the stack can use routing functions. However, the first time you
upgrade the Cisco IOS Software release, all units will be required to run the same SMI or EMI version as the master.

note in the above document it stated on the same release even if they SMI and EMI

Can I upgrade only two switches to EMI IOS and leave another two switches to SMI IOS?

No you can't.  Here's the reason:

1.   Firstly, the entire stack has to be using the SAME FEATURE SET because you can't program, for example, BGP routing on an IP Base switch.  Remember that your ENTIRE STACK operates as one.

2.  Secondly, presume that you did manage to get this running and you need TAC's help.  They'll see what you've done and, most likely, raise their hands up in the air and tell you that they won't support a setup like that.

3.  Lastly, it won't work because the stack will see that the new members are operating in a higher feature set and during a crash, will cause the entire stack to operate in "limbo" and totally un-responsive.

It is better to upgrade your stack to IP Services or DO NOT USE the IP Base software.