Replying to Memory Card Trouble

I used my Go Pro HD Hero this past week for the first time. I am using a Optima 4gb memory card. Everything has worked great but last night my go pro said FUL so I downloaded some of the videos onto my computer to free up space but after deleting majority of the videos off the card my go pro kept saying it was full when I only had 1gb of video on the 4gb card. I'm guessing that the videos arn't being completely deleted when I delete them off the card. Has anyone else had this trouble? I ended up just dumping all the videos and putting the memory card into my camera and formatting it and it works fine now. I was wondering if anyone knows my mistake or a solution to why it would still say full when it wasn't.