I have an Eden cabinet D115XL4 with one 15" 4 ohm speaker and no horn. I want to pair this with my D210T 8 ohm cabinet.
I am using an Eden WTX264 head which is minimum 4 ohm.
Can I simply replace the 4 ohm speaker in the cabinet with an 8 ohm or do I need to do something to adapt the cabinet.
"Can I install a 8 Ohm speaker in an 4 Ohm cabinet?"
No, because once you remove the speaker from the cabinet, the cabinet has no Ohms rating.
Cabinets do not make Ohms. The speakers in the cabinets do.
"Can I simply replace the 4 Ohm speaker in the cabinet with an 8 Ohm..."
Technically you can remove the 4 Ohm speaker and put in an 8 Ohm, now making it an 8 Ohm cab, technically.
Operationally is a whole other matter.
Cabs are designed to work with the speaker intened to be used in the cab. There is a nice little dance going on between the speaker and the cabinet.
You change the speaker and you change the dance. There is a small chance that all will be OK. There is a bigger chance that you won't like the results.
If you are familiar with cabinet modeling then you already know the ins and outs of what you propose to do.
If not, it's a crap shoot. In that case I would recommend just buying another 8 Ohm cab, preferably one that matches the 8 Ohm cab you have now.