No way to know what part "4" is... different parts diagrams have different numbering. If you're talking about the FAQ document's 1908 Luger diagram, it's the middle toggle.
If you're talking about the breech block (in front of the DWM toggle within the toggle train), or the area around the DWM toggle's axle hole, you'll need to replace the part and possibly others that were damaged.
These parts all contribute in an additive way to establishing proper headspace and breech lock.
They are hand fit, and must be properly fit to shoot safely and reliably. Sometimes you luck out with used parts, but not always. Occasionally you will find complete toggle trains for sale as well.
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 Igitur si vis pacem, para bellum -
- Therefore if you want peace, prepare for war.
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