As to the two younger children, if they were adopted then in the eyes of the law you are a legal stranger to them and you won't be able to get them back unless there was some problem that would cause the adoption to be invalid.
As to the two older children, it appears that the father was given custody but your parental rights are intact. In that case, you would be able to petition the Court to modify its orders to grant you custody -- but in order to do so you would need to be able to show that there has been a "material change of circumstances," and typically the courts will not find it to be sufficient if the only alleged change is an improvement in the non-custodial parent's circumstances.
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