Article 4 technically isn’t in Liverpool yet. They are talking of bringing it in but nothing has been done as of yet.
The areas they are talking about are L6, L7 and L15.
It will restrict permitted development of a residential dwelling to an HMO of up to 6 bedrooms. Currently you can turn a house into an HMO of up to 6 people without the need for planning because it’s under the 7 bedroom limit.
The reason they want to restrict these HMOs is because they believe that either:
1. The high concentration of HMOs in that area is bad for crime rates (which it generally is) and
2. HMOs are being tenanted with bad tenants that bring bad problems such as drugs and prostitution.
Point 2 above is what residents in Kensington are complaining about to the Council however I would put money on to say that 99% of all HMOs in Kensington have been strategically converted to target students.
Once they introduce Article 4 into those areas of Liverpool then you will have to submit a planning application to convert any house to an HMO and it will most likely be rejected as that’s why they introduced it to start with