some local authorities are better than others are providing help and support. Ours seems (compared to many) to be very good.
To adopt four together was a huge undertaking and even without any specific special needs, adoption generates it own additional needs. Its quite hard to get approval to adopt four together for exactly this reason. A friedn of mine was approved for 4 about 4 years ago and apparently was only 1 of about 4 couples in the country approved for a sibling group of up to four at the time. They went on to adopt a sibling group of two and she has said she was so glad they hadn't gone ahead with a group of four. As it is the adoption of one of the siblings has disrupted due to his behavioural problems. The information they got in advance was pretty good though I don;t think anyone was trying to cover anything up - its just at the point of the placement it really wasn't that obvious what effects their early abuse left on the children. Support from teh LA they adopted through wasn't great but wasn't non-existant either.
It is true that you can have children with additional needs regardless of how they come into your family however it is significantly more common in adopted children (in fact problem in any child who have have a poor start in life) but this is generally well covered on the preparation course (at least it was on mine) - to the degree that we were all kind of expecting our childrne to have some degree of attachment, ADHD etc problems. The reason that many social workers are reluctant to approve multiple placements at one time is exactly for the reason that the risk of additional needs and therapeutic parenting (is that the term?) is much higher in adoption and it helps no-one if the parent doesn't have the experience/skill/time etc to deal with this. A disrupted adoption is worse than no adoption initally in many social workers opinions so IME they are EXTREMELY reluctant to approve people for larger sibling groups.
I guess the problme is that whilst it might be covered on teh prep course, it hard to imagin the difficulties of dealing with multiple children with extra needs particularly in your position where you ar eleft on your own to deal with it.
I'm really sorry that things have turned out this way for you, but most people will not be in the position of parenting four adopted children alone and many people can successfully deal with one child at a time even if they came with additional challenges. Some poeple will aslo find their local authorities are more supportive than yours sounds.
I'd recommend looking through teh SN boards as they may be able to provide you with some good advice on how to extort help out of your LA, I'm sure you know that they are obliged to provide post adoption support but I have no idea personally how you try to enforce that - Adoption UK may be helpful.