babytink - from the additional info you have given about this girl craving attention and throwing fits if you do anything with your younger child, and seems to see your H as a threat as he comes between you and her. I note you have fostered teenagers before, so must have been lucky with most placements as teenagers are notoriously difficult to foster.
As you probably know the chronological age of many of these children in the care system does not correspond with their emotional age, and so they are usually functioning at a much younger age emotionally (sometimes at a much younger age than would seem possible) their development can often be "arrested" at a time of particular trauma. I obviously can't say anything about this girl but she may well be functioning around the 8/9 year leve (which is a complete guess)
I am wondering to be honest if this is the right placement for the girl, nothing to do with you as carers as you are clearly very fond of her, but she sounds like she is so needy (as abused/neglected children almost always are) that she does not want to share you with anyone. As I said before these children have an insatiable desire for love as they were denied that usually in their formative years. It is impossible to cater for all their needs, as to a greater or lesser extent their early life experiences will affect them negatively to a greater or lesser extent through the lifespan.
This girl has to compete with your 4 yr old and another placement (what age is that child) and naturally cannot have all your attention. Seems like she has been going through a honeymoon period (which you will of course know about) and the honeymoon is coming to an end!
I think her talk of suicide and notes are her way of saying she is hurting and wants more from you - all of you in fact. Your description of relatives visiting and your girl crying in her bedroom suggests that she maybe felt left out (after all you were interacting with visitors) and her loud crying was her way of saying "I need you more than they do" or somesuch. Obviously these thoughts aren't going on at a conscious level and these kids cannot process the hurt from their past.
Does the LA have a specialist teenage scheme (as most of them do now) and I wonder if she would be better placed where there are no younger children with whom she has to compete. I remember a girl very like yours, and eventually she settled with a single female carer who could give her all the time she needed. Please don't think I am criticisig you - far from it- I have spent many years of my working like championing the wonderful work that foster carers do for troubled children.
You don't need me to tell you that you must think of your own family too. I know another move would not be good for the girl(and of course all LAs are struggling with insufficient foster carers) but you do need to think of how you and your family are being affected.
There are some comments on here about why soc workers are not doing more for this girl, but as I'm sure you know, they are all overloaded with cases and the experienced ones will have seen emotionally abused girls like yours many times before. There are no "quick fixes" for these children and sws simply do not have the time or resources to cope with all children in the care system. At least your girl has a counsellor and a psychiatrist. Is your link worker someone in whom you can trust.
Maybe talking to other foster carers who foster teenagers may help too.
Someone asks why it would be inappropriate to phone Samaritans, who would most likely advise you to talk to social workers, which is of course the right thing to do.
Do hope you can get something resolved, but please take care of yourself. You are not responsible for the girl's state and neither are the social workers. Sadly many many children are damaged by their birth parents/carers and thereis no easy solution.