"She used to play here four days ago."
Translation:चार दिन पहले वह यहाँ खेलती थी।
My overimaginative brain made a whole story on this sentence in a second, There was this young girl who was cheerful and played in the parks but she got diagnosed with cancer and she died at young age and her parents are reliving the memories where she was okay playing just 4 days ago. Just a simple sentence induced this thought and a moral that I quote, "Make the best memories with your loved ones before they become a memory." Maybe this got too heavy, sorry for that
"She used to play here four days ago" is grammatically incorrect in English. In general, it is a bad idea to use "used to do" as equivalent to the past continuous tense ("was doing"). In this sense the correct sentence is "She used was playing here four days ago" or "She played here four days ago."
The Hindi sentence is incorrect too. 'खेलती थी' is the imperfective past tense (used to indicate habitual actions in the past) not the past continuous which would be 'खेल रही थी'.
You can't add a time like this with the past imperfective. This sentence should instead have been something like चार दिन पहले तक वह यहाँ खेलती थी - 'She used to play here until four days ago'
The other sentences you suggested would be translated into Hindi as:
She was playing here four days ago - चार दिन पहले वह यहाँ खेल रही थी।
She played here four days ago - चार दिन पहले उसने यहाँ खेला।
Native English speaker here, fully acknowledging how inconsistent English is. True, what is said and what is written very often do not align. So, to clarify, no one will say "She used was playing here four days ago" as you stated. That just doesn't make sense. However, we would say the second statement "She played here four days ago". That would be said more often IMO than "She used to play here four days ago". However, this statement, while grammatically flawed would still be spoken. Clearly, we are not given the context around the statement (as is the case for many phrases in this course) but it is a likely phrase to be spoken depending on the conversation. Perhaps the girl is a missing person, and the police are making enquiries with nearby residents? But I agree, it's a phrase that appears awkward by itself. नामासते et до свидания!