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.
Section 5A contains a defence - see 5A(3) - but the defence will not be available where 5A(4) applies. The defendant has an "evidential burden" to adduce evidence to raise an issue with respect the defence. If that is successful then the court must assume that the defence is satisfied unless the prosecution proves beyond reasonable doubt that it is not - see 5A(5).
The introduction of this offence was preceded by a Report of the Review of Drink and Drug Driving Law June 2010. (285 pages pdf). The Transport Select Committee considered the matter in the Session 2010-11 - Drink and Drug Driving Law - 1st report session 2010-11. (148 pages pdf).
Explanatory notes to the new legislation are available.
Please also see Inside DVLA - New Drug Driving Law comes into force in March 2015 This link contains further links to material aimed at pharmacists etc.