Police investigating the deaths of the Ding family of four in Wootton, Northamptonshire in April last year, arrested a 22-year-old woman from Southend, Essex.
The arrest was made in Plumstead, London on suspicion of conspiracy to assist an offender.
She was interviewed and later released on police bail.
Three people who were arrested on Wednesday, also on suspicion of conspiracy to assist an offender, have also been released on police bail.
They are a 54-year-old woman who was arrested in the Radford area of Coventry, a 39-year-old woman arrested in Gloucester town centre and a 25-year-old man arrested in the Islington area of London.