This is a common misconception, the 4th dimensional creature is still transiting through the 4th dimension, your “dangling their feet in” analogy is a closer approximation with what is happening, they can still interact across the 4th dimension, allowing them to very specifically “break” many of the rules of existence imposed on three dimensional creatures.
Blob wise, that is exactly what is happening, the blob has one or more gigantic bodies, and it extends countless pseudopods from it’s body into our dimension, specifically into the bodies of living creatures, and it uses these pseudopods to interact with and modify those creatures to meet it’s needs.