When your signup is accepted you will replace a botteam in one of the lower divisions in your country. In your case you will be placed in D4 or D5.
You are placed in D4, there are positive and negative things here.
D4 is economic wise better than D5, you will generate more income and your fanbase will be larger. Since it's pretty hard to promote in BB (you need to be the best of 16 teams) it's better to start in D4 than in D5. Another thing is that you are likely to be in a active competition when you league is higher. You will have less botteams in your league, and imo it's more fun to be in an active league, f.e because of the more active division forums.
On the other hand, you will face stronger opponents, which might be demotivating since you will lose a lot in the beginning.
I think your goal should be to try to not relegate. Set up a nice training program with some young prospects so that you'll have a great team which can compete for the title in a few seasons.