No such thing as early release for "good behaviour" in the UK. The mandatory release date is at the half way point of their sentence, so they must be released at the half way point of 4 months.
If they commit a further offence whilst in prison then that would depend on the offence and whether it's dealt with under the court system or prison law. Days can be added for an offence if the prison deals with it.
They will have a Home Detention Curfew (HDC or "tag") release date at some point, but it will be upto the powers that be as to whether that is applied. Because they are non-violent offenders who own their own homes etc then the likelihood is that HDC will be given.