Fajardo & Vieques
 

Would travelers recommend 4 nights in Fajardo and 1 in Vieques, or 3 nights in Fajardo and 2 in Vieques?

Depends, where are you staying in Fajardo? Vieques is awesome, where are you planning to stay in Vieques?

There is little reason to stay in Fajardo except as a stopping off point for Vieques or Culebra. In Fajardo, I would suggest the Fajardo Inn.

We're staying at the Fajardo Inn and plan to use it as the hub for trips to El Junque, Juquillo, the Nature Preserve, and whatever else we discover. Our main reason for the visit to Vieques is the Bio Bay, a little snorkeling, and am not certain what else. This is a first trip, so we just don't know where we might prefer spending a little extra time.

This is just me, but I would spend most, if not all, of my time in Vieques. You can visit the rainforest and other main island sites either on your way to/from Vieques or as day trips. Vieques is a special place, which is going to rapidly change over the next several years. Fajardo (in my opinion) isn't.

Use Fajardo, as you've planned, to visit El Yunque, Las Cabezas nature preserve,and maybe to take a catamaran day trip. I wouldn't stay more than 2 or 3 nights there. Luquillo and Seven Seas beaches are nice, but if you're also going to Vieques, you can get more than your share of great beaches there.

If it were me, I would do 2 nights in Fajardo and 3 in Vieques. That's assuming you're up for several days of beach bumming, relaxing, and not much more. If, on the other hand, you want to experience some San Juan nightlife, stay longer in Fajardo. In that case, you could use one of your nights in Fajardo to drive over to San Juan for a nice dinner, some live music, casino, etc.