The British navy used Watches lasting 4 hours... I imagine the Hindi came from a similar military background... However, in the Navy, the First Watch was 20:00 to 00:00, probably the first chance of a surprise attack after sunset... (they have a special name for Second Watch though, it is called the Middle Watch)
Interesting. There is a lot of minutiae of time divisions in Indian culture (which have become obsolete/forgotten). The Punjabi poet Surjit Patar wrote a poem that begins, "My mother tongue is dying, word by word. My mother tongue is dying, sentence by sentence." After that, he begins a list of all sorts of time-related words that used to be said (presumably in the agricultural environment). You can hear it here, from the opening lines at 0:38 through his list of time-words which ends with the audience applause after he adds, "Time is up for all these words." https://www.youtube.com/watch?v=dya9E8hG2mk For instance, /amrit velā/ is the time really early morning, like 3-4 pre-dawn hour, the "nectar time".
I also found in a Punjabi dictionary (checking quickly) a word, /sāit/ meaning "a measure of time about 24 minutes"! Maybe there is a Hindi equivalent.
I am familiar with the 4 hour watches because I am an amateur sailor. This system was called "watch on, watch off" in the merchant sailing system, because it meant you'd be on and off duty alternating every 4 hours and would try to catch sleep in the off times. I don't know why the British Navy system would call that hour "first" watch, but I'd speculate it's because it is the first full watch after the pattern recommences after the "dog watch", 16:00-20:00. The dogwatch is split into 2 2-hour watches and the 2 halves of the ship's crew get their schedules flipped around after the dog watch. Dog watch is also supper time and leisure time for all the crew. So after eight bells at the end of the dog watch, it's like a new cycle starts.