Help- must choose (southern Portugal or Southern Spain)

Transport links between Portugal and Spain are not great, so unless you fly, it will be easier to stay within Portugal for such a short extension. You maybe lucky with the weather, but you could also get quite a bit of rain in mid October.

Southern Portugal i.e. the Algarve is more focussed on golf and beach holidays. Whilst much of the Spanish coastline also appeals to beach seekers, there are also cities such as Jerez, Seville and Granada to visit. You may have left it too late to get tickets for the Alhambra in Granada.

If you have to make your way back to Lisbon, then I would stay in Portugal, but I would head to Evora, not southern Portugal or hire a car if you haven’t done so already and visit Tomar, Obidos and Alcobaca. If you want the Algarve, head to Tavira, which is the town that has been the least affected by development over the past 30 years.

The Portuguese eat at “normal” meal times i.e. lunch at 12-2pm and dinner from 6.30 to 9pm. The Spanish eat lunch at 2-4pm and dinner from 9pm, often finishing nearer midnight, which I dislike.

I love Portugal, but not the beach oriented Algarve. Stick with Seville and/or Grenada.

No simple answer, need a lot more info. Tell us about where you have been before (been to either Spain or Portugal before?), the context of your trip, what kind of things you like/don't like. Most importantly, what comes after this 4 (or 3) "extra" days? Are your flights home booked yet? From where? Can they be changed?

It's certainly true that Portugal and Spain are not as well-connected and many first time visitors expect, so from an efficiency standpoint, sticking to Portugal probably makes sense. But it really depends on all the other details.

See an overview of other Portugal destinations at https://www.ricksteves.com/europe/portugal.