Lisbon will be the last leg of my European trip (will be flying in from Paris). Wondering if 4 full days will be too much? Will a day trip make sense, if so, to where? Is Sintra a place better saved for a multi-day visit?

As for lodging, can $50-60 a night get me a decent 2* room or apt (clean is the key for me).

4 full days will hardly scratch the surface of what's available to see and do. Your budget is rather slim, but who knows.

Sintra makes for a good day-trip, but you could possibly spend two nights there.

4 full days won't be too much but it's not too little either. We did Sintra as a day trip. It's really worth a visit but I wouldn't stay there for multiple days.

Four days will just get you started. A day trip to Sintra should be plenty. Click on my name for my Lisbon TR. The Residencial Florescente should still be affordable: http://www.residencialflorescente.com/index.php?lg=en

Thanks.
I just booked Residencial Vila Nova for 65 USD (50 euros) per night. Not bad. No breakfast but hotel looks new and good reviews.

I'll see whether to go to Sintra after getting there. Maybe there's too much to see in Lisbon still by the last day.