Teremari... my first thought was "Terra Mariana" (the official name of mediaeval Livonia). Of course, having a coin nicknamed "Mediaeval Livonia" does make much sense--especially if the coin is from 1764. So... maybe not.
Quote: "Fransvannes"Was the Maria Theresia thaler (https://en.wikipedia.org/wiki/Maria_Theresa_thaler) used in the Baltic provinces at the time? Could this have had the value of 4 rubles in 1764?
For the first question, I would say most definitely. After the capitulation of Livonia and Estonia, those two places were not using Russia's currency like they were supposed to, but rather continued to use a Thaler-based system. They relied on foreign coins as their main supply, and with the Maria Theresia thaler being so widely circulated, that was more than likely one of the types to circulate there.
With that being said, the Livonese was introduced as a Russian version of the Prussian Thaler. One Livonese was set to be (wrongly) equal to 96 Kopecks, with 96 Kopecks being equal to 0.96 Rubles. So, to answer the last question: the Maria Theresia thaler would not have circulated as a 4 Ruble piece.
As stated in an above post, 4 Rubles would be equal to around 2 Dukat, so if there was a Maria Theresia 2 Dukat piece that widely circulated, there is a possibility the Teremari could be that one.
Although, geographically speaking, the Duchy of Courland was rather close to Livonia and Estonia, and in 1764, they did in fact issue a
single-year 2 Dukat piece. The Teremari could
possibly be that one, although the main problem with that theory is that the Courland piece does not have Maria Theresia on it (unless, of course, Teremari does not actually stand for Maria Theresia.

).