Answer:
4.1. I decided to befriend the new girl from whom I received a gift, which I really appreciated.
4.2 We met the old man who moved to our town last year while we were in London, where we visited the museum and parks together.
Step-by-step explanation:
In the first sentence, we must use a relative pronoun to talk about the girl, since the information about her is repeated in two clauses. We can choose between "who" and "whom", but the latter is the best option because of the preposition "from". Then, we need a pronoun to refer to the gift. In this case, "which" is the best choice, since it is used to refer to objects or animals.
In the second sentence, we use "who" to substitute the information about the old man. Then, because London is a place, we employ "where" to connect clauses that talk about it.