As usual I am being incredibly stupid; I have the four hint words and can see the relevance of the first three but the fourth just doesn't work for me. There are only 4 words of the correct length that I haven't used and and none of them seem to have any connection with the 4th hint/.
I have the four words and have applied the down clues' positions as I understand the preamble. Is the result a tune I am meant to recognise? I am totally unmusical and may have to resort to finding the grandchildren's toy xylophone at this rate. I've tried putting the notes into Google but to no avail. I must have got something wrong I think.
Planks, if you translate what you have into A-G, then google the 4 groups of 4 resulting from action of the numbers of the down clues, you should find it. It's been around a long time and likely you hear it every day.