Hi OP (do feel free to pop over to the adoption board any time, if you need any more advice or explanations)
I've adopted 2 older children myself, so I can explain why SS have age gap rules
Firstly, I'm sure you do have a lot to give a child. And you CAN adopt - nothing you've said would rule you out of adopting a child younger than your DD
However, adoption, first and foremost, is about the adopted child. Social services won't take any risks, either with the children in care or with your own DD, and adoptions 'out of birth order' are very risky
The reasons for the rule are that:
Older children have usually been through a great deal. They have probably been abused/neglected and lived through a lot of disruption and chaos. Consequently they may have a lot of issues - my older children certainly have been through a lot and have a lot going on. Things ranging from aggression, raging, destruction of furniture and other objects, lying, stealing, sexualised and sexually reactive behaviour, controlling behaviour, attachment issues, academic delay, social delays, behaviour problems in school...and so on
You won't necessary know what the childs issues are when they move in, hopefully you would be told but some things are just not known about
It does put your little DD at risk. Her safety - physically, sexually, emotionally - is paramount
With an older child who doesn't have major issues with aggression or sexual behaviour, any behaviours they have are things your DD would likely copy, which would be challenging
Aside from that, ANY older child, no matter their issues, will need a huge deal of time and attention. They are likely to be emotionally younger than their years. You might wind up with two children competing for attention, one who desperately needs it because of their situation, and the other who needs attention because she's a young child. Except the adopted child will need MORE attention and time
Older children definitely need homes where they have buckets of love, BUT they also need homes with no younger children in, for everyone's protection and for the older childs benefit so they get to be the baby, have all (or most of!) the attention without taking too much away from another child
Your DD will be 4 quite soon, and then it seems they have no objection to you adopting a little one. Unless you definitely are done with 'baby' in which case in a few more years, your DD might be 8/9 and you could adopt say a 4/5 year old. With eyes wide open about what this might entail
Best wishes