A 68-year-old man with long-standing hypertension is on maximum tolerated doses of an ACE inhibitor, a calcium-channel blocker and a thiazide-like diuretic. His clinic BP remains 158/94 mmHg. His serum potassium is 4.1 mmol/L and eGFR 70 mL/min/1.73 m². According to NICE, what is the preferred fourth-line drug?