Praise be to Allaah.
It is not permissible to deny the unity of judicature, because it is one of the types of Tawheed, but it comes under the heading of unity of worship (Tawheed al-‘Ibaadah) with regard to the ruler when he rules according to sharee’ah. But with regard to this concept (Tawheed), it comes under the heading of the unity of Lordship (Tawheed al-Ruboobiyyah), because the Ruler is Allaah, may He be exalted.
The Lord Who is in control of all things has to be the Ruler, so it comes under the heading of Tawheed al-Ruboobiyyah as far as rulings, commands, prohibitions and control are concerned. But with regard to implementation and acting upon it, a person is obliged to follow the rulings of Allaah, thus it comes under the heading of Tawheed al-‘Ibaadah in this regard.
There is no reason to make it a fourth category of Tawheed, because it is included in the three existing categories. There is no need to add further categories. The matter is easy to understand anyway, but if a person wishes to study the issue of judicature (al-haakimiyyah) separately, he will find that it is synonymous with other concepts, but that is fine.