Episodes 4, 5, and 6 are clearly poorly done upscales taken directly from the DVD, this was not a ripping issue, the episodes appear this way on Amazon Prime Video Japan. Most likely, Senkosha (or whoever currently holds the rights to Iron King) has lost the masters for these episodes or they are damaged, leaving the DVD as the only available source for “remastering.” In any case, it is truly unfortunate that not all episodes were remastered.
Information:https://ultra.fandom.com/wiki/Iron_King_(series)