I would say that 2nd (rue Etienne Marcel and surroundings), 3rd (rue Charlot) and even 11st arrondissements (rue de Charonne) are now more trendy, even if rue des Francs Bourgeois (between 3rd and 4th) and surroundings are great spots for fashion boutiques, cafés and restaurants.

OK, but the responder (Stimpy?) implied that the Marais was NOT hip at all...
The 3rd and the 4th overlap into really one neighborhood, IMHO. Really almost the entire 2 districts from the Place des Vosges to the Beaubourg to Republique are full of hot spots. Don't know that area around the 2nd, is it just a couple of blocks? The 11th? Hmmm. Not bad, but not the HIPPEST.
The new up & comer is the 10th around Canal St Martin, but it's got a way to go to catch up to the Marais...

There are a couple of fun bars in the lower Marais, but that does not comprise the entire 4th arrondissement. To me it is full of hokey tourist places. The most fun I ever had in the 4th was at a private party in the Hotel de Ville.

But to each his own.

Hmmm. I don't recall ever seeing a "hokey tourist Place" in the Marais which to me means a place with cheap "I love Paris" Tshirts and brass little Eiffel Towers. (Those I've seen in other areas, in Montmartre, on the Champs, and on the Rue de Rivoli near the Louvre). There are mostly hip, trendy shops in the Marais, along with restaurants frequented by locals. Of course there are tourists, Paris is the most touristed city in the world! But I love the 4th.

During your next stay in Paris, visit:

In the 2nd:
visit Passage du Grand cerf (especially la Corbeille, great vintage design shop, Haute définition, glass gallery)
Have a tartare poilé at Etienne Marcel, restaurant designed by M/M
Shop at Martin Sitbon rue du Mail (design shops in all the street and famous chez Georges restaurant)
Walk around rue Tiquetonne (Wesc), shop at Kabuki (certainly the best men shop in Paris)
Buy some food at incredible G. Detou
Have a spa at Le Klay or at Nuxe

In the 11th:
Have a brunch at Réfectoire, a drink at very fresh Place Verte, design by Adict
Visit Edition Limitée, Walldesign, Union Libre, Galerie Anatome and of course Galerie Patrick Jouin
Have dinner at Unico, if you are a great meat lover
Have a last drink at great Ave Maria (trendy restaurants all around if it is full) or Café Charbon
Dance all the night at Nouveau Casino (great music, extraordinary sound)