The second two examples have broken as an adjective. The get example has it as a past participle, and the be example is ambiguous.
The window was broken by the ball. [past participle]
The window was pretty broken. [adjective]
Try the same series with a verb that has a past participle which doesn't share a shape with an adjective.
The window was cleaned.
The window got cleaned.
*The window seemed cleaned.
*The window ended up cleaned.
The passive voice is made up of be or get and a past participle, whereas 'linking verbs' take a noun or adjective. Be can mark passive voice or serve as a 'linking verb' depending on context. Get can have a 'linking verb' interpretation, especially with comparatives.
The window got smaller.