Some of the additional jerseys that have been awarded variously over the years for a consideration of the 'fourth award':
1) Metas Volantes / Intermediate Sprints:
This is the most immediately obvious one of course, but at the moment is rendered a bit pointless by a few factors, such as the importance of the intermediate sprints for the maillot vert, the disincentivisation of breakaway activity and the way that such a competition usually works in short stage races which gives rise to a perception of its lack of value, where somebody will get in the break on stage 1, go again on stage 2 and open up an unassailable lead. Or, worse, something like Coryn Rivera in the 2018 Women's Tour, where the bonus seconds were crucial for the GC so she ended up winning the competition without ever leaving the péloton, the precise opposite of what it was intended for.
2) Intergiro:
A cult favourite of many, the Intergiro basically has a mid-stage point where time is taken and it is then treated like a legit GC, so that the breakaway riders can duke out their role as the king of the breakaway. It would also to an extent rather supersede Super Combativité, however. In latter years, however, we would see breaks pulled back for sprinters to get bonus seconds in the Intergiro, Massimo Strazzer was particularly notorious for this. It would need its own sprint that was not related to the time bonus sprints or the green jersey intermediate in order to be effective, I feel.
3) Combiné:
The Combiné is, if we're honest, both the most 'classic' and the least worthy inclusion because it isn't much greater than the youth jersey in terms of not really having a competition for the jersey itself, with current parcours trends and the domination of certain riders in it, it probably ends up being what it ended up as in Spain in latter years, the jersey worn by 2nd on GC because the GC winner invariably won it. From its introduction in 2002 to its final inclusion in 2018, only three times did the GC winner not win the combiné as well - 2002 Heras won it coming 2nd to Aitor González, 2003 Valverde won it coming 3rd, and 2012 Valverde won it coming 2nd to Contador. 3/17 is not a good record.
4) Italian style Combiné:
In 2006 the Giro tried an interesting twist on the Combiné, awarding points after each stage for the top positions in each of the major competitions. This meant for the classification it was just as useful to be top of a lesser competition for a while as it was to lead the GC because you wouldn't just inherit a strong position in the classification by default. It did, however, have a bit of a goofy wrinkle that meant that Paolo Savoldelli by winning the prologue - which included a small hill - got the lead of all 3 classifications off the bat, and because all stages paid the same points in those days, that kept him up near the top of both the points and GPM standings in week 1 before his fellow GC contenders came into the hunt for the classification, so despite finishing nearly 20 minutes down on Basso he was classified the "most complete" rider in the Giro and wore the jersey from day one to the end, which seemed silly.
5) Activity Classification:
An old Ostbloc specialty, this was an awesome classification, but I'm not really sure if it could work - or even be operable - in modern cycling. Essentially, the classification would give points for the first few riders past intermediate sprints, mountain summits, and at the finishing line - but only if you were in a group of 10 or fewer. You would gain bonus points for each 10 seconds your group finished ahead of the next group on the road as well, to incentivise pushing on deeper into the stage and adding time to your adversaries. I love it as an idea, but I'm not sure of its practical application in 2024.