>>15695i'm not sure how committed you are to this 400 calorie thing, but in case you don't take the other anons advice to just not do it, i can try to give some starvation advice from first-hand experience (this is my experience at least, results may vary)
first: it will usually take a huge amount of willpower to only eat that much for any extended period. it may seem easy at first, but the longer it goes on, the more you will want to eat something carb/fat heavy. it might help long-term to designate like one day a week where you can eat a meal of something sugary/high-carb/etc. just to give your body something to look forward to
second: at least initially, you will lose weight at a very very rapid pace. i'm in a completely different weight bracket (5'10 80kg), so my weight loss numbers wouldn't rly help you, but you could expect to lose anywhere from ~1-3lbs per week and (in 4 months) could probably get your weight from 49kg to like 37-40 (your progress would eventually slow down a lot, since you already seem like you're pretty skinny and probably don't have much excess fat or muscle to lose, but it would be very quick at first)
third: (at least in my experience) your body will feel like a nightmare afterwards. if you fully commit to 400cal/day, your body will definitely adapt to it after 4 months, and will not like when you reintroduce real portion sizes and caloric intake. i'm talking like even like one burger or one bowl of cereal causing stomach cramps, chills, maybe occasional vomiting, you get the gist. also, since ur body has 'learned' to live with 400cals, any excess (even a low intake like 1200cals) will end up being stored as fat. so unless you plan on eating 400cals forever (dying) or very very slowly upping your caloric intake again (like increasing it by 30% every month back up to 2000, taking another like 4-5 months) then be prepared to feel like shit for an extended period of time
again, im not a health expert so take whatever im saying with a grain of salt (including your food, add salt to it since salt is 0 calories and promotes hydration which will make you feel more full)
despite you thinking that you need to lose weight (you dont! please dont develop an eating disorder), you might not even have enough fat storage to support 4 months of starvation. you could very realistically and accidentally starve yourself into being chronically lethargic, weak, and depressed then possibly suffering from refeeding syndrome and being hospitalized
be safe!