Grouping few of many sourceSets having exactly same configuration

Lets say I have the following sourceSets:

sourceSets {
    flavor1 {
        assets.srcDirs = ['repo-assets/flavor1']
        res.srcDirs = ['repo-res/flavor1']
    }
    flavor2 {
        assets.srcDirs = ['repo-assets/flavor2']
        res.srcDirs = ['repo-res/flavor2']
    }
    flavor3 {
        assets.srcDirs = ['repo-assets/flavor1']
        res.srcDirs = ['repo-res/flavor1']
    }
    flavor4 {
        assets.srcDirs = ['repo-assets/flavor2']
        res.srcDirs = ['repo-res/flavor2']
    }
}

If you notice flavor1 and flavor3 have same srcDirs and so does flavor2 and flavor4.

Trying Possibility#1

I was trying to figure out if there is a way to avoid the redundancy by using something like this:

sourceSets {
    flavor1, flavor3 {
        assets.srcDirs = ['repo-assets/flavor1']
        res.srcDirs = ['repo-res/flavor1']
    }
    flavor2, flavor4 {
        assets.srcDirs = ['repo-assets/flavor2']
        res.srcDirs = ['repo-res/flavor2']
    }
}

The above does not work (already tried). Looking for something similar so that i can just provide a common set of sourceDirs for a set of flavors. Anyone tried doing something similar and can provide some pointers?

Trying Possibility#2

Does the name of sourceSets need to be same as that of flavors?

Can i name the sourceSets separately and then map them to productFlavors like this?

productFlavors {
    flavor1 {
      sourceset = "src1"
    }
    flavor2 {
      sourceset = "src2"
    }
    flavor3 {
      sourceset = "src1"
    }
    flavor4 {
      sourceset = "src2"
    }
}

sourceSets {
    src1 {
    }
    src2 {
    }
}

Trying Possibility#3

Can the sourcesets be dynamically assigned via tasks somehow to achieve the same stuff?

UPDATE

Douglas's answer sort of helped me get very close to what i was looking for eventually (reducing the code in build.gradle). He used Possibility#3 above. Thanks Douglas! Any better alternative from bounty hunters is still welcome (something closer to possibilities #1 and #2 above). If nothing comes up the bounty is Douglas's already when the period ends as I've accepted his answer. But still will remain optimistic about finding a better alternative.