MyGCSE Science contains over 50 different Khan-academy-style videos covering different aspects of GCSE level science. The website is designed specifically for the AQA Core Science, Additional Science, Biology, Chemistry and Physics courses but you will find many of the videos will be suitable for any KS4 science course. The site is free, but they do plan to offer additional resources for a small fee.
They are lecture-style and so might not be suitable for all learners, but for additional support for students or for revision purposes it would be a handy website to have bookmarked.
The site hosts all the videos on YouTube, so you’ll need to check it’s not blocked before planning to use in a lesson.
See all the videos at : http://www.my-gcsescience.com/