Good job with the backup, Salvio! Regardless of the storage device that it's stored on, it's highly recommended to always have at least two identical copies of your data. As I mentioned those 4 pending sectors are not necessarily bad yet. Indeed, you could try repairing them by performing the low-level format feature from the DLG tool. If you want more information about it and how to do it, you can refer to our knowledge base article here:
http://products.wdc.com/support/kb.ashx?id=6RBEqT
Keep in mind that it would erase everything, so make sure you have the data copied somewhere else beforehand. However, what did the Extended test from the Data LifeGuard say about this particular S.M.A.R.T. attribute?
The DLG utility can help you re-map & block these sectors, but if you're still concerned about the WD My Passport, you should do a warranty check on our website:
http://products.wdc.com/support/kb.ashx?id=W2gwGr
If it's still covered, you should contact our customer support and send an RMA request to get a replacement.
WD Support contacts:
http://products.wdc.com/support/kb.ashx?id=oxRrUd
As for your new hard drive:
Since you want to be able to take the HDD in and out of the enclosure whenever you want to troubleshoot something, it's better to get an internal drive with a third-party enclosure. If you get an external HDD and dismantle it, you'd be voiding the warranty and you might also encounter some access issues without the enclosure, due to a hardware encryption on some models.
Either way, I don't think you should give up on your WD My Passport just yet. Give the WRITE ZEROS option a try and then run the QUICK and EXTENDED tests again, see what the SMART attributes will show.
Hope I helped. Keep me posted.

SuperSoph_WD