We fit two adults and two teens in a standard Scamp
13’ front bunk model. My wife shares the 45” dinette bed with our younger daughter, sleeping head-to-toe (lots roomier for two that way). I take the lower front berth and our older daughter takes the upper. She is still within the 120# weight
Four fit without modification, unless you’re thinking about a front bath model, maybe? In my strong opinion, if you want to sleep four people with a bath, go with a 16'. Without a bath, a 13' is fine. Perhaps you could elaborate on the need to save weight
... I thought you had a Sienna with a 3500# rating?