meggle I know just what you mean...
there was a previous thread I started about my own 3.7 yo who cannot spend a minute alone & prefers to spend his time velcro'd to your leg. (He also drives his nanny bonkers.)
He follows his adult companion(s) from room to room constantly, asking questions. If you open a cupboard, he is right there next to you wanting to know what you are picking up and demanding an explanation!! Maddeningly he will also not play by himself, even if in the same room as you, is murder to get him to draw, model playdough etc. His toys might as well be in the charity shop. He just constantly demands your attention and conversation and will play up if he doesn't get it.
On that thread a lot of people concluded this is just a personality type, a bright mega-extrovert sort of thing, but that doesn't make it much easier to live with. Recently dh decided to get harsh and tell him to go and play in his room on his own for 30 minutes. He came down the stairs every 1/2 minutes saying 'I've done it!' It will be a long road I think to change this one.
Like you I have a dc2 imminent so I think that my patience is going to snap even more often with my little emperor...