Sydney man, 84, missing for four days

Police are continuing to search for an 84-year-old man who hasn't been seen since leaving his northwest Sydney home more than four days ago.

Serious concerns are held for Youliang Lin, who suffers dementia and does not speak English, since he left his Castle Hill home on Cooper Court on Friday morning wearing a black jumper and long black pants, police said.

He's slim with grey hair and about 175cm.