You do get a bonus in the fourth year but only if you haven’t taken the first lot of money out.
Example: you put in £50/month for two years. Balance - £1200, get £600 bonus paid into your bank account. Leave the balance in your Help to Save
Do the same for the next two years and end up with £1200 + the next £1200 + £600 = £3000
But if you took out £1000 when your first two years was up, and continued saving £50/month, you’d only then earn a bonus on whatever you saved over £1200, so you’d have to pay in £1000 and then only earn the bonus on the final £200 - your last bonus would be £100.
It’s to discourage people immediately taking all the money out and blowing it, I assume - to try and get you in the habit of long-term saving.