Your bad teeth are likely to have abscesses developing on them. The treatment for abcessess is either root canal treatment or extractions.  The crowns of the teeth or the spaces left aftr extractions would need to be restored.  To have 4 teeth treated by root canal treatment and crowned would cost in the region of £1400 per tooth.  If the extraction option was taken I am unable to give you a costing until I have assessed your mouth.  Hope that helps.