It sounds like it's on Sleep Mode. Sleep Mode is accessed when tapping the Flex several times in a row. To take it off Sleep Mode, tap it a few times until you see only one light blink. FYI, you can tell when your Flex is on Sleep Mode when you see one light on either side light up.
Basically what Sleep Mode does is it monitors your sleep. When you check your dashboard the next day, you can find out how well you slept, how many times you turned over, how many times you got up, and even how long it took you to fall asleep. The 4 light pattern may not be abnormal when plugging in/resetting the device. I see it on mine when charging with a very low battery or restarting. If you see it at other times, then it may indicate a firmware or hardware fault. With only one light blinking while charging, I suspect that your battery is very low, possibly low enough that the device doesn't function (and appears dead) when not plugged in.
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It sounds like your device may not be charging properly. Have you tried the cleaning and troubleshooting steps for a charging problem that are pinned at the top of this forum? If following those steps doesn't help the situation, you'll need to reach out to to further investigate the issue. Let us know how you make out. I am having this same thing happen to me right now. I know exactly what 4 lights you mean and I read all the comments. I have had a fit bit for 1.5 years so I definitely know it isn't in sleep mode.
Fitbit Blinking While Charging
I also know that it isn't that I need to 'get to stepping' like another suggested (haha). I charged my fitbit on Thursday evening to a full charge and by Friday evening it completely stopped syncing and when I tapped the fitbit no lights came on at all. I was out of town without my charger since I had charged it Thursday night so of course I couldn't try charging it until now. I just got home and plugged it into my computer and it did the same exact thing as what happened to you. 4 lights, 2 on each side, blinked twice, then 1 quick blink of the 1st light. I unplugged it then plugged it back in again, it did the same thing but this time it kept blinking on the 1st light as though it is charging.
Did you ever come to resolve on this? I will see what happens with it appearing to charge as I am typing this. But if you have any hindsight advice it is greatly appreciated! Mine is doing this, too and I've self-diagnosed that it's a problem in the connection between my fit bit and the charger. I insert my fit bit into the charger, which is plugged in to my computer, see the four lights, and then nothing happens. I wiggle it about a bit, and it seems like the connection takes and it begins charging normally.
Fitbit Blinking Lights
Right now it's plugged into my computer and the end with the fit bit is propped on my keyboard, so that the connection stays in place and it continues to charge. The other day, I didn't have it in this position and left it charging, only to find it had barely charged - again, my guess because the connection was bad. I'm thinking I need a new charger.