My kids are 4 years apart. Just so happens.
It is GREAT!
My eldest, my daughter... adjusted well. BUT it was because, while I was pregnant, I PREPPED her... not just concentrating on 'my' pregnancy.. but including her.
I explained in child terms, that my tummy grows, and each month we took photos together of her with my tummy. She loved that. She loved, her baby brother, in utero.
She sang to my tummy, talked to it.... I told her I will get tired and have to take care of my 'tummy'... and we napped together. She'd even pat my rear as I went up stairs, to 'help' me.... when it got harder for me to get around.
I took her to all my prenatal exams... which my Doctor encouraged.. and he even taught her how to use the Doppler heart monitor on my tummy... and she LOVED that.
I ALSO... explained about what a "baby" is.... that they cry/wake/mommy breastfeeds... just like she did as a baby. But that it is not her 'job' to be 'perfect' or to suddenly grow-up... .but to be HERSELF... and to ALWAYS tell me how she feels... good or bad. And that I will be there for her.
We made up special 'hand shakes' and nods... so that she could feel 'bonded' with me and special.... and like a 'team.'
I told her... to NEVER feel, that I am 'too busy' with baby... because SHE is important too... she just has to tell me. And its OKAY.
I fully prepped her, while pregnant... for what is going on and what to 'expect'.
I told her that she will ALWAYS be "my first baby...." always.
I also told her... that her things are hers... and I will respect that. That I do NOT expect her to 'share' everything. To a child, this is important.
Its okay.
I told her... that just because she is the 'oldest' that she is STILL a child... and she does NOT have to be suddenly all perfect and flawless... that I will NEVER use her as an 'example' for her little brother... nor will I "expect" things from her that she is not yet capable of....
I had a baby-shower for my 2nd pregnancy... which friends did for me.
PREP your eldest child... BEFORE the baby comes home. Let her ask you questions and to express her feelings about it.
It is not only you, that is having a baby. She is too....
My kids, are super close and like 2 peas in a pod.
My daughter "loved" her baby brother, even while I was still pregnant. She did not feel threatened by it... I prepped her during my pregnancy and before baby was born.
all the best,
Susan