I think it's more a case of the minimum wage increase above the inflation for several years in a row during Lula/Dilma governments. Also I believe the average amount being paid is much higher than only 4% of the minimum wage.
Also Bolsa Familia is not a true UBI program, there are several rules for people to make part of it and only 14 million families receive the amount. Even when it was created and was more relevant it was just a little bit of help for poor families to buy some food and make sure the kids could stay in school, as the amount was never enough pay for your rent for example.
It is just a % of the minimum wage, and the minimum wage in Brazil itself is already a sad joke.
It's a combination of both. If I'm not mistaken the average is about 16% (170 BRL), most poor income of families in Brazil have 4 people
But I made I mistake, when the benefinit started the limit person per family was 3, the calculus was different, it was 24% of minimum wage (by the time) + only 7% per person who were either child, teenager, pregnant or nursing mother. Hence the limit you could get was 95 BRL, roughly 45% of the minimum wage instead of 80% I thought
However the accumulated inflation for the last 18 years is 294%, hence the threshold of Bolsa Familia should be 375 BRL (around 38% of minimum wage)
And you are totally right, even 375 BRL for a 5 people family would be already far bellow anything necessary to make a living, 70% of beficiary families have at least one person with a regular paid job