Adjusting condition in field calculator in QGIS

Have you tried something like :

CASE 
  WHEN "Surface" >= 30000 THEN 4
  WHEN "Surface" < 30000 AND "Surface" >= 10000 THEN 3
  WHEN "Surface" < 10000 AND "Surface" >= 5000 THEN 1
  ELSE 0
END

Additionally to the @J. Monticolo's answer, you may try using if-statement

if("Surface" >= 30000, 4, if("Surface" >= 10000 and "Surface" < 30000, 3, if("Surface" >= 5000 and "Surface" < 10000, 1, 0)))