You could use them together if that's all you have, but there will be differences. The text below describing the two speakers was taken from the Identifying Your Advents sticky in the speaker forum. Even though the 3 and 4 use different sized woofers, the biggest difference is the efficiency ratings. The /4 will be noticeably louder than the Advent 3.
The advent /3 is a great small bookshelf that still has the classic look and sound. It uses the same cone tweeter as the model 2, but only one instead of two in an array. The plus to the /3 was the amazing bass response for such a small box. However, its downfall is its terrible efficiency (85db / watt) which makes it power hungry, but at the same time easy to bottom out if you turn them up. Most will use them in an environment where good sound is preferred but excessive volume is not (IE an office or bedroom)
The woofers were 6" with butyl surrounds or treated cloth surrounds (depending on year produced)
The advent /4 was the solution to replace the old Advent /3 --which although was a good speaker -was inefficient. The woofer and tweeter were used in later designs. There were also two grill designs like the /1 -one rounded grill and one sharp. It's an 8" two-way design.
