Section 4 is the offence of being "unfit" to drive (i.e. the person's ability to drive is for the time being impaired). Section 5A introduces into the law a specific limit offence for "specified controlled drugs." (Controlled drug has the meaning given to it by the Misuse of Drugs Act 1971 s.2).
The difficulties involved in proving impairment due to drugs means that section 4 of the 1988 Act is not often used in drug driving cases. While section 5 of the 1988 Act makes it a separate offence to drive or be in charge of a motor vehicle with an alcohol concentration above the prescribed limit, no similar offence existed prior to the new legislation for drugs.
The applicable limits are set out in The Drug Driving (Specified Limits)(England and Wales) Order 2014.