Hi Moto
You need to write to your landlord terminating your part of the tenancy, send the letter recorded delivery and keep a copy. If you have to give four weeks notice I would suggest paying the landlord your share of 4 weeks rent (although he could come to you for it all as you are joint tenants). You will then have no further financial liability, ask the landlord to view the property before you return the keys so he can see you have caused no damage etc
One word of warning pet, make sure you have somewhere safe to go when you leave, if you apply to your local council as homeless you will probably be found intentionally homeless and only given advice not temporary accommodation or a new home.
Hope everything works out for you