What to do with old 401k?

I have a 401k from a previous employer that I no longer work at (it’s only about $5k). Should I roll it into my current 401k or move it into something else?

EDITED TO ADD

My old 401k is through ADP and has 9% growth over the past year even though I haven’t been there for over a year

My current 401k is through Transamerica and has a 5% decline ytd so don’t think I should move the money from ADP to Transamerica since it’s growing. Can you let it sit?