The original claim isn't that characters break the fourth wall, just that they are aware that they are fictional. In TV Tropes terms, this is leaning on the fourth wall or lampshading. (Not coincidentally, both use the same line from Twelfth Night as a page quote!). 674
Many of these examples "could also be interpreted in other, non-immersion-breaking ways", but I'd argue that an audience watching a play on a stage with a literal proscenium would not fail to notice that the actors are commenting on the play in progress.
Here are some examples from the Leaning on the Fourth Wall page:
* Twelfth Night: "If this were play'd upon a stage now, I would condemn it as improbable fiction" (Act 3 Sc 4)
* As You Like It: "All the world's a stage and we are but players." (Act 2 Sc 7)
* <PERSON>: "How many ages hence shall this our lofty scene be acted over in states unborn and accents yet unknown!" (Act 3 Sc 1)
* <PERSON>: "Life is but a walking shadow [= actor], a poor player [= actor] who struts and frets his hour upon the stage" (Act 5 Sc 5)
And some from the Lampshade Hanging (Theatre) page:
* Most of the female characters pretending to be male tend to lampshade the fact that they are male actors playing a female characters disguised as male.
* <PERSON>'s "Speak the speech I pray you" monologue can be seen as a combination of putting a shade on the common Theater techniques of the era, and a Take That! against the overuse of it.
* In Henry VI Part 3, things are going badly for the Yorkists at the Battle of Towton. The leaders gather and exchange poetic speeches about how badly it's going, that being how characters do things in these plays. <PERSON> reproves them thusly: "Why stand we like soft-hearted women here, / Wailing our losses, whiles the foe doth rage; / And look upon, as if the tragedy / Were play'd in jest by counterfeiting actors?"
And some examples from my own brain:
* The most straightforward example is characters who explain their actions (past and planned) to the audience in soliloquies, although this is more of a theatrical device than fourth-wall breaking. This includes <PERSON>, <PERSON> and, of course, Hamlet.
* The Merchant of Venice: "I hold the world but as the world, <PERSON>; / A stage where every man must play a part, / And mine a sad one." (Act 1 Sc 1)
* The Tempest: "These our actors, / As I foretold you, were all spirits and / Are melted into air, into thin air: / And, like the baseless fabric of this vision, / The cloud-capp'd towers, the gorgeous palaces, / The solemn temples, the great globe itself, / Ye all which it inherit, shall dissolve / And, like this insubstantial pageant faded, / Leave not a rack behind." 627