Totally agree with Twinkly and Honoraria and beautifully put by both.
Would just add, my 3.10 yr old dd1 (who sometimes does not like her 12 month old sister one bit) seemed to get a lot of relief from a chat about what was making her angry.
Intially, this was dd2 getting lots of attention - but more recently its been about dd2 grabbing toys. This is probably more relevent when your 4 mo starts crawling, but: we had a big chat about space...hers, dd2's, shared/family space. Now she's clear that downstairs is family space, and up for grabs, but that her own room is hers, and dd2 hsa to be invited in - or can be asked to leave (via a grown up).
I did a lot of telling dd1 that I understood how furious she felt at having her space invaded, that I'd sometimes felt like that when I was little and had a baby brother (this works especially well, she sort of listens harder) and we worked on finding solutions that worked for her (eg putting her dolls house in her room where dd2 can't get to it) and figuring out how she can let us know she needs space/time out from her sister before things get too much for her.
eg she will occasionally say 'mummy, I feel like hitting M' and I'll say 'well done for telling me', and ask her what she needs (its usually space or attention). She's never actually hit her, as a result.
And after a whole year, they're finally starting to play together - especially in the bath, where they can 'gang up' at splashing Mummy