Hi sorry to hear you are feeling miserable. i think the advice you have been given in the previous thread is good, and i think (no expert here) that yes it is fairly normal behaviour for a newly older brother and 4yo.
try not to let people pigeonhole him in terms of 'he's always been good....whats gone wrong...' type thinking as all children have stages they go through easier than others. my oldest was a real screamer as a baby but an absolute delight as a toddler.vice versa for my youngest. This type of 'labelling' may seem positive, and indeed, be true at the time but it means the child has expectations placed on him which he does not understand and cannot live up to.
its great he shows his baby sis how much he loves her most of the time and clearly throwing things at her is not on, but most siblings have survived the jealousy/rivalry albeit with a few scars -my older sis held my head under the bath,I threw a fork at her eye (not at same time)
keep going with the time out thing and really praise him when he is being loving toward his sister.
Try not to look too much into this, like squeakytoy says its normal jealousy. Your son may not connect you being upset with what he has done to his sister.