Thanks for your kind words! (and also thanks for occasionally spreading a good word about GraphHopper 
)
With 4 buckets and time limit of 1800 the biggest isochrone should be equivalent of 2 hours of walking
Buckets work differently: it will split the 1800 seconds into 4 similar sized buckets i.e. 450 * 4. See the documentation of the web service: graphhopper/docs/web/api-doc.md at master 路 graphhopper/graphhopper 路 GitHub
Does this resolve your issue or do you still observe problematic behaviour?