You need the bartender to refill your glass up to one fourth. Is there a word for the quantity in English? In my native language we say tahpeyk which literally means the bottom of the shot glass. The word is also used casually for liquids other than alcohol. For example, one can say, 'Pour me a tahpeyk' when a friend asks him if he wants any tea while playing cards.
You can also ask the bartender for "a finger".
A finger more of the same.
But not of something poured into a shot glass. This request assumes a wider-mouthed glass with relatively straight walls, not a small conical glass.
In English-speaking countries, alcoholic spirits (such as whisky) are normally sold by the shot (or measure), the size of which varies from country to country. In the USA, it is normally 50ml; and in the UK, it is either 25ml or 37.5ml, depending on the establishment.
shot noun [C]
a small amount of an alcoholic drink:
a shot of whisky
measure noun [C]
an exact amount, especially of alcohol:
If one wanted two measures of spirits (in the same glass), one would ask for a double (or large) [type of spirit].