10 Days in Catalonia

oleg, all options are okay, I mean MIGHT be okay. Depends greatly on the weather, end of September. Girona and around might be convenient but ...
Best would be to choose shortly before, depending on the fortnight forcast.
A. San Feliu de Guixolsis a pretty small beach town to hang out some days. And since you are driving it's easy to visit some small towns such as Besalu (tourity after 10.30 ) and idyllic Peralada. Or stay here (in P.) for some days and visit Figueres and Cadaques (+LLanca) from there.

B. Of course more south-west might be warmer.
Coastal road to Sitges is gorgeous. Sitges is worthwhile a visit and a stay. 'Driving around small towns on the coast': not really enjoyable. Better go inland then, for instance N420 to Mora d'Ebre and sidetrip to Miravet, back to the coast on the C44 and visit the small gem Tivissa.

C. 4 days is enough. Getting around the city is doable (if you don't mean by car)

But as said: better wait till circumstances are more obvious.

I've spent the 2 first weeks of October of last year in southern Catalonia, on average the weather was good (and certainly a few degrees warmer compared to northern Catalonia, which will make a significant difference that time of the year) with the odd day with some raindrops. Swimming in the sea was still done.
Sitges is nice (though touristy) and Tarragona is very worthwhile visiting, but apart from those i agree the interior is more interesting here imho.
Apart from the area mentioned by Arizona i can suggest places such as Siurana, Poblet Monastery, Montblanc…

I like plan B best, as you would be able to visit both a large city and take in some countryside.

That said, there aren't really any small towns on the coast worth visiting, or where you can hobnob with the locals. Pretty much all of the Spanish Mediterranean is an overbuilt concrete tourist trap.

Instead I'd recommend heading inland. Towns such as Morella, Albarracin, Molina de Aragon, Teruel. Do a Google Image search. You have 10 days which is plenty of time

Nobody knows what the weather will be like in first week of October as it can be quite volatile then.

I like your suggestions Mredman but i think they are a bit far for the OP (and not even in Catalonia anymore) since he will only have 4 days to play with and wants some relaxing time as well. OP will stay in Barcelona 6 out of the 10 days he has...

I missed the 4 days bit!

I can only comment on option C. The weather should be good, unless exceptional circumstances, tourists still go swimming in Valencia at the end of Sept/beg of October. It´s not a huge city, 4 days are more than enough to see everything and get a good feel of it. It´s easy to get around, especially if you use the metro.

The weather end of September and the first 2 weeks of October is typically still summer here - people (well tourists, as pointed out above ! :-) ) still go to the beach and swim. The biggest difference with summer is not that much the temperature but the fact you might get a gray or rainy day or two.

Obviously more to the south it gets warmer - Taragona is warmer than Barcelona, and even more so Valencia.

Girona is good for a day only, but there are other towns around that are interesting as well - eg Besalu, or you could cross the border into France to Couillure - if you have a car ! Of your 3 options I like this one least.

Tarragona is nice, but also for only a day. Some excellent suggestions of small inland towns to visit above.

Valencia is nice, 4 days is good and more not needed. Nice city centre, some amazing modern architecture, good beach, fine dining. Also a pretty different feel than barcelona. I like this option best, but someone above suggested that after 6 days in a big city you might want some countryside !

I´ll go with the combination of options B and C, it will also depend if you are travelling by yourself, budget, wether if you are travelling on party mode or you are on a more realxed-sightseeing mood. You can spend a couple of days south in Tarragona (like many on this thread have mentioned before, it might be warmer South Catalonia than North by the Costa Brava). You can visit Tarragona (amazing food), and visit some nice places likes Cambrils or Salou, and then go furthers South all the way to Valencia.

Costa dorada is a really amazing destination and by that time of the year is not so crowded like in summer hish season time.

Valencia is a really nice city and you can see everything there is to be seen there in a couple of days.