This may be a dumb question...

Is there any harm in hooking up a powered monitor to a powered mixer? I'm assuming you can keep the levels down on one or both and keep the total power output at a managable level.

Our main PA mixer (16 channel, not powered) only has 2 monitor outs. We are planning on using 4 monitors and I have a separate 4 channel powered mixer. So, my thought was plug the 4 channel mixer into the 2 monitor outs and use that mixer to connect and tweak the on stage monitors. One of the monitors is powered however. Is there any harm in hooking that into the powered mixer, or is there a better way?

Are you talking about having a mixer for front of house and a mixer for monitors?
Also powered monitor to powered mixer is fine, you just don't need to run the monitor through an amp like you would if the monitor wasn't powered
Mixer for monitors. 1 powered stage monitor + 3 passive stage monitors out of one 4 channel powered mixer.
I assume what you have is a four channel mixer with a stereo amp built in. 3 passive speakers and an active monitor (with a built in amp). This means you have 3 amps and want to drive four speakers.

Can you drive one of the passives off the amp in the active monitor. Then you can connect one speaker to each of the stereo speaker outputs, alternatively you'll have to have two speakers coming off one channel from the mixer.

To drive the mixer make sure you use the monitor output from the main mixer and check whether it is a stereo or mono output, you'll also need a line level output to the powered monitor, you will probably find one from the second mixer, don't feed a speaker output into the mixer!