I have a similar age gap and now DS is two it is properly awesome, they rough and tumble on the floor together, chase each other around and they get a lot of pleasure from each others company. They also care a great deal about each other and there is a lot of spontaneous affection. DD helps DS a great deal and protects him to a certain extent. Just this evening DS climbed off his sister's head (play wrestling rather than attempted murder) and kissed her nose before telling her she was his best friend in the whole world.
They are also learning a lot about compromise, negotiation and where to poke each other to get the maximum reaction. I am learning about crisis management, dividing myself in two and remaining calm in the face of adversity/whining in stereo/working out who smacked who in the face with the sonic screwdriver first.
DD is at school most of the time which is great but holidays take some getting used to and when DD has friends around DS can't really join in easily so there is some tension there. The age gap means they have to find a middle ground when playing and DD often gets frustrated with DS.
The first two years were hard- DS had reflux and lactose intolerance and poor DD was neglected. Ds got frustrated that he couldn't keep up with his sister and discipline was really hard to manage. DD couldn't understand why she got punished for biting/hitting and baby DS didn't (we didn't ignore it but we didn't make him sit on the naughty step for example).It was hard finding things for physically active DD and DS when he got too big for the sling. He got really frustrated just sitting in his pushchair watching her run and play and he had much less patience with play parks so DD only got half her accustomed time.
Naps were hard as he wanted to sleep around school run times and I was constantly dragging him out of bed.
But it was lovely having a child who could entertain themselves when DS was a baby; DD could be set a task, be negotiated with or bribed, do arts and crafts etc while I was dealing with DS. Jealousy was more mental than phsyical, DD got quite angry and morose but she did occasionally take it out on him with a swipe of her hand (and once pushed him down the stairs but we won't dwell on that as he was absolutely fine and she wept for an hour).
Washing increased by another load a day, it was tricky at first to fit in housework but both kids now help out a little which makes it easier. PLus they play independently which is magnificently brilliant! DD and DS both do drawing and arts and crafts much longer if there are two of them at the table, although there are squabbles.
Four year age gap is great. Especially when it comes to illness e.g. they both had chicken pox when DD was 5 and DS ten months. DD was happily applying her own cream and distracted with DVDs whilst I wrestled with DS. When they both have a sickness bug they stagger it so I have a chance to get some towels and bedlinen dry and DD can be trusted to vomit in a receptacle, leaving me free to chase around DS.