工具 (gōngjù) is the general word for any tool, instrument, or means — physical (hammer) or abstract (education). 仪器 (yíqì) are precision instruments for scientific/technical use; 器械 (qìxiè) are equipment/apparatus, often medical or mechanical. 手段 (shǒuduàn) is an abstract method or strategy, sometimes with a negative connotation of trickery.