Quais são os problemas de equilíbrio mecânico em uma corrida jogável com mais de duas mãos?

Simply allowing more arms is overpowered

Allowing a PC to use a two-handed weapon with a shield is overpowered indeed. The game wasn't created with that in mind: either your character use a two-handed weapon to deal more damage or use a shield with a one-handed weapon, trading-off the higher damage for better protection.

By simply allowing a PC to have four-arms, without limiting what it can do with those will surely break the game. The PC will be able to ignore a lot of "exclusive options", trade-offs it can simply avoid; the two-handed weapon with a shield is just one of them.

Spellcasting with a sword and shield (without War Caster), wielding a shield with a spellcasting focus and dual-wielding, using a hand crossbow with an shield, and any other situations that can't happen when you only have two arms are some of the trade-offs a four-armed creature can avoid.

So, how to balance it?

If I were to create a Thri-Kreen race, I would limit what it can do with those extra arms. A good starting point is to limit what interactions can be done with those extra arms. The Trunk feature from the Loxodon (Guia do Mestre da Guilda para Ravnica, page 18) gives us a good starting point:

You can grasp things with your trunk, and you can use it as a snorkel. It has a reach of 5 feet, and it can lift a number of pounds equal to five times your Strength score. You can use it to do the following simple tasks: lift, drop, hold, push, or pull an object or a creature; open or close a door or a container; grapple someone; or make an unarmed strike. Your DM might allow other simple tasks to be added to that list of options.

Your trunk can't wield weapons or shields or do anything that requires manual precision, such as using tools or magic items or performing the somatic components of a spell.

Of course, the thri-kreen extra arms aren't like a trunk, so it needs tweaking. I would reduce the number of pounds it can lift, and well, a thri-kreen can't use arms as a snorkel. I would also remove its ability to grapple or interact with a creature; their extra hands are small. Instead, I would allow them to have "one additional interaction with an object or environment, for free, during your turns".

The most important limitations of extra arms are in the second paragraph. I would never touch those, because that's where the majority of problems lies with extra hands.

As Matthieu M. pointed out in the comments, a good explanation for why shouldn't a Thri-kreen extra arms be limited, is that the creature's brain is limited, and can't coordinate so many tasks with the required precision.

A greatsword with 18 strength deals an average of 11 damage (2d6+4).

A shortsword with 18 strength deals an average of 8.5 damage (1d8+4).

If we want to balance a character that can use a greatsword in place of a shortsword, one simple solution would be to apply a -3 modifier to all melee attack damage. This could be justified narratively by explaining that, although the thri-keen has more arms, they are individually not as powerful.

The problem with this approach is that it pretty much obliges a thri-keen character to use two handed weapons in all cases.

If you want to give the extra arms some additional utility, perhaps add a boon to interactions, for example the ability to draw/sheath a weapon with a single interaction.