The slaughterhouse race number 4 seems to be blocked by a door.How do we open it?

One opens it simply by winning the third race. I don't know if you had to win the first and second race as well, since I did those each playthrough anyway. It seems logical enough that the first three races had to be won before the door to the fourth opens, in my opinion.

Alright,got it and already got all pearls,animals and Mdisks.
How much time do I have to wait until those guys send me an email with a mission?

it should say once you get all the pearls that you get a new mdisk. try it out and it contains a pearl game. once you get all the animals, you should get an mdisk with all your animal pics on it. if you did both of these things, check your mdisk inventory and they should be there already. collecting all the mdisks doesn't do anything really. though there is an unlockable mdisk if you haven't gotten it yet. you need to log onto the darkroom website it tells you on your current save (check here on the forum if you got problems) and type in your code provided. if you're far enough along, you'll get a code for the safe in the acuda bar. inside it contains an mdisk of the "air hockey" game.