Rosie you really need to discuss this with your dentist.
Unfortunately you are definately confused.
In any quadrant you only have a maximum of three molars (one of which is known by lay people as a wisdom tooth).
If you are saying that you have three there already + a wisdom tooth that makes four. (It is possible to have an 'extra' molar but quite rare).
So assuming you don't have an extra molar I assume the tooth you are referring to is the second molar (known as UL7).
Whether the cheek would be affected depends on your soft tissue/muscle profile however usually there are few problems as a result of having a single molar removed.
Depending on your age a wisdom tooth (or any tooth behind one that is extracted) may drift forward and partially close the space.
The remaining teeth have to take more load during function but the loss of one molar does not usually cause adverse effects.
There will be loss of some alveolar bone which will shrink in the region that the molar is removed from.
The benefit of having root canal treatment is clearly that you get to keep the tooth.
It may require a crown in order to adequately restore the tooth to function and also to seal off the root canal system.
Remember however that whilst there is a fairly high success rate for root canal treatment (depending on anatomy, age, previous RCT, number of canal etc..) it is not 100% and if it fails, re-root canal treatment or extraction may be necessary.