There are too many unknowns here for us to help you out. We don't know which switches you intend to use afterwards (locations), what access you have to joining the circuits together, or what the possibilities of joining them would be without knowing of your personal setup.
Your setup, roughly as described to us is...
<--- S --- S --- Kitchen Lights
<--- S --- S --- S --- Adjoining Room Lights
The arrows (<---) would be your final wire running back to the source. The (S)s are your switches. And the (---) are wires between each. Now, this doesn't have to be physically wired in this setup nor does it have to be the only load on the circuit (no outlets or anything else?), but it would be how things are internally wired to control the lights. So no matter where the power actually comes in or where the switches are located in relation to the lights, this will be how the connections are made.
Then, if there is nothing else being fed from the kitchen circuit and you're intending to use only the 'adjoining room switches', you could simply disconnect and remove the breaker, wiring, and switches feeding the kitchen lights. Find the first light in the kitchen in sequence that receives power from the switch and add a wire between it and the power feeding the 'adjoining room' lights. The easiest way would be if you can get above the lighting, such as up in your attic, but where ever is accessible will work. Reconnect the hots, neutrals, and grounds and you should not be able to control all of the lights from those three switches. Your new layout would look like...
<--- S --- S --- S --- Adjoining Room Lights --- Kitchen Lights
However, again, this goes off of the assumption that you're fine with controlling them all from just the adjoining room location. If you're hoping to split the switches up between the kitchen and that room; then it'll all come down to how everything is currently wired, which switches you want to use and which to remove, and what wires are ran between each and from where. To get a complete answer, we would need a lot of information. However, a licensed electrician would be able to figure this all out on his own and get the job done for you.
Lastly, as I've had to mention recently in another post, four way switches are only used between two three way switches. You might have a four way switch setup in the adjoining room, but that does not mean that all three switches are four ways - two are three ways and only one would be a four way.