You wont hear enough of the band. You should assume you wont hear anything unless you have the other band members in your mix.
Here's what I would do with your setup (whilst keeping costs down)
Plug your vocal mic into channel 1 on your Behringer 1204 mixer.
Use a balanced jack to XLR cable to connect the aux output from your 1204 to your FoH mixer. (Set aux send at unity on the channel aux send and master aux send). As far as FoH is concerned, just consider your mic as being directly connected as usual.
Plug your bass DI into channel 2 on your Behringer 1204 mixer.
Buy a pair of cheap condenser mics (e.g. something like a Behringer C2), plug them into channels 3 and 4 on your mixer - and put them on stands L and R of the stage so you can pick up whats going on on stage. I'm not sure whats happening lead singer wise, but you may need to experiment with mic placement to pick up everything as you need (maybe even having your mic pointing towards a wedge if they are monitoring that way). Make sure you turn on phantom power on you your desk so the condensers work.
Ensure all aux sends are down on all channels apart from the vocal mic channel.
Plug your Xvive U4 into the L out on the desk. (Being mono is fine as long as all channels are panned centrally or all to the L)
In order to set your mix, just ride the faders (remembering the master output needs to be up also).
Doing it this way gives you some control of your feeds by enabling you to add in some EQ (cut lows) to taste.
Have fun!
What desk are you using for FoH? Or is the Behringer the FoH? If it is the FoH, disregard the above...