When you were booking the holiday you were told that another balance was due 4 weeks later.......even though youve now received something else, I really feel you should contact them and pay the amount required, after all its what you thought you would be doing in the first place!
Even if they wont cancel the booking if you DONT do that, you must have planned to have it available when required, and surely its better to pay it and get it out of the way? They wont charge you twice for it and it will be less to pay shortly before you go.
In other words, maybe someone has made a booboo with the booking, but surely its better to pay this and have peace of mind about it.