How do you cure a kitten's loneliness?? Please help?

I have a situation which I need help with. Ive been taking care of a stray female cat and her 4 1 month old kittens for 1 1/2 months now and I got more and more attached to them as the days went by and they became a part of out daily lives. I have a big backyard in which the kittens used to play and frolic with each other like all kittens do and I assume that they loved each other being siblings (all females btw). Everything was dandy, 3 routine meals a day, a nice soft garden to sleep in, plenty of play time etc. Heres were the problem starts.....I woke up today morning to find all but 1 kitten missing....i assumed they were around because i have a big backyard and they wake up by between 8 to 10. But there was no sign of them anywhere. That 1 kitten left behind was mewing loudly almost frantically outside so i fed it thinking it was very hungry ,but after eating it resumed with its sad mewing. Its been doing this continuously all day. I think its very lonely and is calling out to its sibling trying to find them. Its gotten to the point were its running around in frustration and scratching and biting anything and everything it sees. I've searched for the other 3 kittens everywhere including neighboring compounds but I can't find them and with every passing minute that 1 kitten gets more and more frantic. My heart bleeds for the little one. These 4 kittens were a bunch and very attached to each other, but now 3 are gone and 1 is left behind. he mother is also here but shes not bothered. I'm getting worried myself, i'm afraid of letting the last 1 out of my sight because there are alot of big stray dogs on the roads near my house. I'm starting to get the feeling they might have killed the other 3 if they wandered out of the yard. Where could the others have gone?? Is it normal?? Can i expect a return in a few days?? How do I calm the last kitten down?? Please help I like cats allot but don't now much about them.......