I went to WDW this October/November when I was about 4 months along as well. We had a great time, and I was glad that we went! I was also worried about what rides I could go on, etc. March should be a good time of year to go, because it won't be overwhelmingly hot. In any case, here are some tips.
1. Stay hydrated... it's really important to be hydrated while you're pregnant, so carry around a water bottle (I refilled mine along the way).
2. Frequent bathroom trips. I never used so many bathrooms at WDW, but all that extra water has an effect.
3. If she feels tired, sit down and take a break. I had to slow down a little from the pace I normally use at WDW, but it's important to take breaks before you get exhausted.
She should talk to her OB/GYN about the rides. Everyone's doctor gives slightly different advice. My doctor said basically to use my common sense when it comes to rides, and gave me the ok for Splash Mountain and Tower of Terror (Luckily, my OB/GYN was really familiar with the rides at WDW). I avoided Space Mountain, Big Thunder Mountain, Mission Space, the Rock N Roller Coaster.