you said you also had a sucking loach, so I assume you have 1 sucking loach, 3 neons and 1 guppy.
Neons need a bigger tank than yours because they need the swimming space. They also need to be in larger groups than that.
For the guppy, I'm not so sure, but I think they are better in groups and would do with a bigger tank also.
I don't know about the sucking loach so I won't comment, I'm not even sure what species you are referring to.
You didn't say whether you did a fishless cycle or not, so I assume you didn't. How long has the tank been set-up, when did you buy the fish, and how often are you are doing water changes? Considering you (probably) didn't do a fishless cycle and you aren't testing your water it is very likely you have got high ammonia or nitrite, which could explain why your scissortail died and why you are now having problems with your guppy.
I'm not experienced enough to tell you exactly what to do, but a 50% water change would probably be a good idea, and you need to return the neons, probably the sucking loach and possibly the guppy (hopefully someone else who's more experienced will come along and give you some more definite advice).
If you haven't cycled your tank, and considering all or most of your fish are unsuited to your set-up the best thing would probably be to return all the fish, do a fishless cycle (with test kits), and use this time to properly research the fish you can have in that tank.