just caught a wee bit of this. sorry you're having a hard time. My ds1 is 4.5 and ds3 is 4 months. I have had problems with him since baby came - more chronic moaning than anything else, wasn't much fun, but he's definitely calming down. don't know what ratio of normal 4 year old behaviour:reaction to new sibling, definitely a mixture, as it is with your ds.
anyway, don't have any solutions for you (sorry) but my jaw dropped when you listed the amount you do with him. It's a bit different with my ds1, as his brother, ds2 is 3 and they're very close, so when I'm with baby, they're usually playing together, but there is no way I spend anything like that amount of time with them, and there is no way i'd suggest you're not giving him enough attention - if anything too much. When they start asking for treats is when they stop getting them - I fell into this trap when ds 1 changed playgroup & wasn't very happy - I'd have a treat for him when he came out, but prretty soon he started asking for it, so I just stopped altogether. Now he occasionally gets one, but never when he's asked.
I wpould do your best to stop feeling guilty, to stop tip toeing around him, and to have very clear boundries which you don't waver. In your original post, when he said "Look what I did Mummy, I ate all the biscuits!" that clearly said "and what are you going to do about it?"
It will get better really soon, just don't take too much crap from him & don't feel like you're not doing enough, you are. Look after yourself too, most imp. - 4 hours sleep is not enough to look after yourself, never mind baby & badly behaved 4 yr old too. It'll all come together quick, don't wory.