1.Two and two are/ is four.
2. I arrived in / at hcm city last week.
3. I hate to see/ seeing animals suffering.
4. I like working on farmers/ farms.
I’m not sure if speakers of AmE would accept ‘is’ rather than ‘are’ for (1), but it is acceptable in BrE. Both options are possible.
2. Usually ‘in’ though in some contexts ‘at’ might be plausible.
3. both possible.
4. farms. One cannot work on a farmer - the person who manages the farm.