looks like some type of juniper tree. They're usually pretty resilient.

This looks like a juniper to me as well from the picture. If I had to guess I would say it is what's commonly known as a Chinese Juniper or a similar nursery variety.

More info on bonsai cultivation of Junipers is here: www.bonsai4me.com/SpeciesGu...perus.html

Junipers don't typically do well indoors as they require full sun (not filtered or shade). I have heard of people keeping certain Juniper species indoors but if they do survive it would take meticulous care and they would likely not thrive and their growth would slow down. Putting the tree next to the window might seem like a good solution but in fact it kills a lot of bonsai because on a hot day all the sunlight is focused through the window and you will burn the foliage, and stress the tree.

If you want an indoor bonsai you should find a species that works well indoors like a ficus. I would say probably 9 out of 10 people that get bonsai as a present will do the exact same thing that you did and put it indoors by a window or something like that and are disappointed by the results. The fact of the matter is that store bought bonsai (affectionately known as "mall-sai" by bonsai people) are often already severely stressed, diseased, or even dead when they are bought, even if they are still green. It may just have taken 4 months to see the results on a species like a juniper.

If you can put this tree or another juniper out on a balcony or fire escape or put up something like a window box (make sure you strap the pot down with wire or something like that so that it doesn't go flying if a good breeze hits it) that would be your best bet.

Some day commercial nurseries will find a good bonsai species you can cultivate happily on a window in a studio and water it once a month for people like us in the bay area who are busy and have nowhere to put an outdoor tree. Until then your choice is pretty much either make use of whatever outside space you can or get a houseplant. Some folks have done amazing things with a ficus or schefflera though, don't get me wrong.