As previously posted, a friend and I are in the process of putting together our prospective itinerary for June 2023 to France, Spain and Portugal.

Due to time constraints we will likely only have an opportunity to visit Lisbon (with a day trip to Sintra possibly) and have been told by friends that have lived in Portugal that we can probably do Lisbon in a short time frame, particularly at the pace we will be travelling at.

We have originally allocated 3 days for Lisbon however we have the potential to add a fourth day.

What are everyone's thoughts on this? Should we deduct a day from one of our locations in France or Spain in favour of more time in Lisbon (or even potentially doing a brief stay elsewhere in Portugal) or stick to 3 days for Lisbon?

Four days in Lisbon would be much better. You need most of a day just for the Belém area of the city. We spent nine nights and never ran out of things to see and do.

Ditto.
We recently, last month, spent 7 nights in Lisbon and never ran out of things to do/see. And yes, the days Belém requies a full day, given the crowds.

Given the choice, four days for certain, especially if one of them will be taken up with a trip to Sintra

I had 3 days , didn’t make it to Sinatra.