Ok, I see where my issue is - I should read instructions better. It is said in the header code injection bit:
"var divNum = new Array("a1","a2","a3"); // at the left you should add a1, a2 etc. for each header you wish to include
// so if you want 4 headers you should add a1, a2, a3, a4 in the format shown
// enclosed in double quotes
...and I did not add "a4" for my 4th accordion. Problem solved.