Trials should only be open to New / Unsubbed users, but I've got it to work for a second time on the same account.
You could also use a new/second Google Account to make it work (ie different email/account)
I'll explain how I think I got it to work for a second time on the same account at the end of this post
So, It's free for the first 4 months, then you'll be charged £9.99 per month. You can cancel without charge at any time before the trail ends. Go into Settings and click Cancel Subscription, you'll be asked why, then taken to your "Google Play Account Store Account" page. You'll see your Music sub, with option to unsubscribe next to it.
Without this link the default free trial is Two Months, but there always seem to be links for Four Month trials out there. Previous "Four Month link" posted 3 months ago no longer works. When this stops working you can search for "Google Play Music Four Months" and limit your search to the previous month. This should find the currently working 'vouchers'
How I got a second trial on same account
Not sure how I exactly did this, or if it will work for everyone.So below is exactly what I did. I initially used the link in a Chrome Browser logged into my main Google Account. It'd didn't work because I'd already had a free trial last year. So I opened another Chrome Window logged into another Google Account. This time it worked, and I gave Paypal for payment.
I obviously wanted to use it on my Android Phone. So I went into "Google Accounts" in my phone settings and added this account (you can log into several google accounts in Android, but be careful to only sync stuff like email/music). With this new account added I opened up Google Music in Android. At the top you can use a dropdown menu to select which account to use. I chose my new account, and it seemed to suggest my old account music cache/history on the phone would be wiped when changing user. I did this. Then I used to drop down menu to select the main account to see what had happened. Once again got warning that switching music accounts wipes Music cache/history, but I did it. Found myself in my main music account, but completely reset, and offering me free trial (defaul 2 months). (note - Playlists/library/cache had been deleted only from phone, and still exist in Google Online account, so they're downloaded again when you switch back)
So went to main account chrome browser and tried the 4 month link, and it worked. Now have 4 month trial on main account (and 2nd account)[br/] So I'm hoping that changing accounts in the Android "google music" player wipes details of your previous trial. Or it could be linked to having subbed in the other account? Or I just could have been lucky. Worth trying because it doesn't cost anything (if you remember to cancel)