First payments on account bill — why am I paying next year's tax already?

Yes, this is completely normal — it's how the system works and it catches almost everyone out the first time. Because your bill was over £1,000, HMRC assumes you'll owe a similar amount next year, so each payment on account is half of last year's bill: £2,000 now (alongside your £4,000 balancing payment) and another £2,000 on 31 July. The good news is these aren't extra tax — they're advance payments that come off next year's bill. And yes, you can reduce them: if you genuinely expect this year's profits to be lower, you can ask HMRC to reduce your payments on account online or with form SA303. Just be careful not to reduce them too far — if your actual bill turns out higher, HMRC will charge interest on the shortfall.