Sahih Muslim Book 17, Number 4221
Abu Huraira reported that Allah's Messenger (may peace be upon him) was asked about the slave-woman who committed adultery and was not protected (married). He said: If she commits adultery, then flog her and if she commits adultery again, then flog her and then sell her even for a rope.
Are 4 witnesses required for slaves?
Well you claim you are not cherry picking, but the first hadith in this chapter in Sahih Muslim says:
When the slave-woman of any of you commits adultery and this (offence of hers) becomes clear, she should be flogged (as the prescribed) punishment, but hurl no reproach at her. If she commits adultery again, she should (again be punished) by flogging, but hurl no reproach upon her. If she commits fornication for the third time and it becomes clear, then he should sell her, even if only for a rope of hair
Which indicates that the pre-requirements for a punishment for committing zina are fulfilled.
Imam an-Nawawi explains in his commentary on Sahih Muslim -see here- saying:
Here and in the follwoing I'm translating from Arabic language, as these translations are of my own take them carefully!.
ومعنى تبين زناها : تحققه إما بالبينة وإما برؤية أو علم عند من يجوز القضاء بالعلم في الحدود
"and this offence of hers becomes clear" means: it became clear by evidence either by eye witness or with whom it is permissible to judge with knowledge within the limits.
Al-Qurtoby explains it as follows in his commentary -see here- on Sahih Muslim:
وتبين زنى الأمة يكون بالإقرار وبالحبل، وبصحة الشهادة عند الإمام.
The zina of the slave girl is made clear by confession and by conception, and by the validity of the testimony according to the imam.
So witnesses are among the necessary pre-requirements for condemnation!
There's one difference of opinion among the scholars, it is whether the slave owner can punish once he has the "evidence" or the imam is necessary to give a final judgement to apply the punishment.
And Allah the Almighty ordered:
Those who commit unlawful sexual intercourse of your women - bring against them four [witnesses] from among you. ... (4:15)
And those who accuse chaste women and then do not produce four witnesses - lash them with eighty lashes and do not accept from them testimony ever after. ... (24:4)
Why did they [who slandered] not produce for it four witnesses? ... (24:13)
It is agreed upon that four (free, Muslim) male witnesses are necessary for a condemnation for zina, based on this verse. However the ruling on slaves has a difference of opinion when it comes to the punishment based on whether the slave was married or not and who was involved in this act of zina. This is also based on the interpretation of "الإحصان" in the verse from surat an-Nisaa'.
Some form of evidence is certainly required as the hadith says:
إذا زنت الأمة فتبين زناها فليجلدها
If a lady slave commits illegal sexual intercourse and she is proved guilty of illegal sexual intercourse, then she should be flogged
However it is not always necessary for the proof to be four witnesses.
In general jurisprudence, the sources of proof for passing judgement include the following:
Witnesses - meaning when trustworthy people testify that they have observed the act taking place. The number of required witnesses can vary depending on the nature of the case, for Zina it is four.
Confession - meaning when the culprit himself accepts that he has committed the act
Clear circumstantial evidence, such as
Knowledge of the judge. This means when he sees and hears the act himself.
There are a few more but I will not list them here as that is tangential to the topic.
As far as cases of Hudood are concerned: The validity of the first two is agreed upon, the validity of the third one is disputed.
The fourth one (knowledge of the judge) is also disputed. The vast majority do not entertain it as valid evidence in most Hudood, but some do accept it. Those who do argue that it is analogous to confession and witnesses, as when the judge can accept a confession then he should also accept what he has witnesses himself as that is more certain and absolute. Further, this applies for free people, there is greater disagreement on the case of slaves who are being judged and punished by their owners.
The Malikis have explicitly stated that an owner can not enforce Hudood based on his own knowledge.
فقال مالك: يقيم السيد على عبده حد الزنى وحد القذف إذا شهد عنده الشهود، ولا يفعل ذلك بعلم نفسه
Malik said: The master applies, to his slave, the hadd of unlawful intercourse and of false accusation of intercourse, if the witnesses render testimony before him, but he does not apply it on the basis of his own knowledge
While the Hanbalis have stated that he can enforce punishment based on his own knowledge:
وإن ثبت موجب الحد بعلمه أي السيد فله إقامته لأنه قد ثبت عند ذلك كما لو أقر به ولأنه يملك تأديبه
An owner can enforce the Hadd on his slave when it is proven based on his personal knowledge as it becomes proven to him just like it is proven through a confession and because he has the authority to discipline the slave (in other matters without requiring witnesses)
— Kashshaaf al-Qinaa’ also see al-Mughni
So, to answer your question:
Are 4 witnesses required for Slaves?
Not necessarily. Evidence is required but it can also be confession or pregnancy. And some rarer madhabs might allow it when the owner himself has witnessed zina taking place.