I wear a Blundstone 7 or 7.5 depending on the style. I wear a US size 10. The Blundstone 7 or 7.5 translates to a US size 10 for me. I am not familiar with the UK sizing. Hope this helps. If you have a store to try on the variety of sizes I would try them on until you find the Blundstone sizing that works for you.
OK...I wear a U.S. women's size 9 or 9.5. Aussie/UK size 7 (the Blundstone boots I bought) is a roomy size 9.5. So I suspect that UK size 4 would be more like a women's U.S. 6.5/7, not a 7.5. But I'm not sure. I would try googling size charts to compare--I'm sure you can find them. Good luck. It is confusing!