CC Madhya 24.291

ajāta-rati sādhaka-bhakta, — e cāri prakāra
vidhi-mārge bhakte ṣoḍaśa bheda pracāra
Synonyms: 
ajāta-rati sādhaka-bhakta — immature devotees engaged in devotional service; e cāri prakāra — there are also the same four varieties; vidhi-mārge — on the path of regulative devotional service; bhakte — devotees; ṣoḍaśa bheda pracāra — there are sixteen varieties.
Translation: 
“Within the category of regulative devotional service, there are also immature devotees. These are also of four varieties. Thus in regulative devotional service there are altogether sixteen varieties.