I would doubt you have anything to worry about. For them it is also "just" $4 (that was emphasis not sarcasm)... If you were to make a habit of it, then they may think that you were trying to cheat them but that doesn't appear to be the case.
If you just e-mailed them yesterday they are probably swamped and will get to it as soon as possible. Phoning is usually faster but if you aren't in a huge hurry then I would wait and save the phone call for someone who is in a hurry. I work p/t for a company that gets a lot of e-mails regarding product questions, cs issues, thank yous, i hate yous, etc...we get through them as quickly as possible but it is hard to get through them all the same day like we would want to. I had a gentleman call yesterday who was mad that we hadn't answered his e-mails yet. I went looking for them after dealing with his issue and he had gone from 0 to 60 in about 2 seconds. He sent one e-mail at 10pm after we were closed, and the next one the next day in the middle of the afternoon. He was just about next on the e-mail list we were going through but he started out saying how horrible our service was because of one mistake on his order (that wasn't made by us actually but our distributor) and the next e-mail said that if we didn't respond immediately he was going to contact the BBB and his CC company and deny all the charges (though he had gotten the most expensive item on his order and was using it.)
That isn't to say you were going crazy asking about your missing yarn. It is just an example of what they are likely dealing with and why they may not have responded yet. Calling is good, but i wouldn't automatically assume the worst. (I never understand why our customers don't just call us when they are as crazed as the guy above was... I guess it is easier to be ridiculous to us by e-mail than it is by phone...lol
I had another woman who e-mailed to tell us we are the worst company she has ever been VICTIMIZED by and how dare we keep her money for 7-10 days after we realized we weren't able to fulfill her order. I went looking and within 2 days of her placing her order, we realized that we were completely out of stock and wouldn't be able to get more until after the first of the year. We e-mailed her immediately and issued credit immediately for her order. we tell everybody it can take 7-10 days because the banks can be very slow putting the money back on the card. apparently she has never had credit issued before. We had already issued the credit but she was just nuts and said that she was going to do everything she could to ruin our business. I am not terribly worried that she will succeed but seriously....
I will never understand how people manage to go automatically to livid without even talking to the people they are angry with...lol