What's an instrument that sounds good with a guitar, a violin and a flute?

I've been composing over quite some period of time a piece for Guitar Violin and Flute, but I recently composed something else for 4 cello's and would like to hear how it would sound on 4 different instruments but am not sure what to use for the fourth instrument. It needs to be an instrument that exists in Sibelius 4 or is similar to one that exists in it. I've been thinking something like either a piano or harpsichord or perhaps a viola... or possibly timpani or bagpipes?