Are These Sentences Equivalent or is One More Appropriate than the Others? (Present Progressive Tense)

These all seem equally "valid" to me:

"I have recently been reading some research papers about..."
"I recently have been reading some research papers about..."
"I have been recently reading some research papers about..."
"Recently, I have been reading some research papers about..."

Are these all equivalent or is one more "appropriate" than the other, and why?
Thanks!