I think if they are the same sex, two for one baths help moms a lot! :) If one is a girl, it is okay to teach discretion but four isn't a biggie really in my opinon. I think my kids took baths together until my daughter was five.
I however due to having a girl and baths not being good for them and all of that, started showers for my kids last year.
I have a four and seven year old and one gets in the shower, gets their hair wet, I squirt on the shampoo, they are capable of shampooing themselves and rinsing in the shower, then I bought them both those sponge/bath thingys and put the babywash on that and they scrub themselves down and rinse. I then help them out of the shower and drying off a bit. I stand outside the shower while they are doing what they need to for added safety.
I turn the water on and off so they don't burn themselves but they do the rest and my son did great at 3 doing this and loves it. It is much quicker, less water and save me big time on the bath situation. I will still let my son jump in the tub if he is messy from school and take a bath but we for the most part stick with just showering every other night (the kids not me, I have to daily! :) ).