I hope your alk is more than 4mg/L! Did you mean dKH? That's still very low, as is your pH, especialy if they are fluctuating.
Here's what I'd do:
Do a big water change.. Like 3 gallons or more just to try and pull out as much of the suspended dead matter as possible.
Hunt around and make sure there's nothing else dead that you need to get out.
Look at a method of raising your pH and alkalinity - there are additives and drips you can employ, all of which have their own followers. Washing soda can be used to increase your pH (careful, adding too much can bring your pH as high as 12 - we're much better around 8.2-8.5), and it can also help boost your alkalinity.
I was going to ask about your live rock but I see from your sig that you have a good amont - you *could* pull pieces out and sniff them, however. If something died on them (like a sponge that was exposed to air in the last little while), that could also contribute to your water parameters.
Decaying organic matter will produce ammonia (test that, too) - but as a byproduct I understand that the bacteria will use up bicarbonate ions.. What that basically means is that bacteria hard at work are going to deplete your alkalinity and thus your pH.
And don't buy sea stars anymore - I just don't hear enough people having any measure of success with the majority of them. They are either a sensitive species that dissolve or bloody carnivores that eat the tank.
