Well, I'd say spend as much time here as you can but 4 days is the perfect amount of time to see all of Lanzarote's main sights and do a couple of fun activities. If you prefer your travel a little slower, want to soak up the atmosphere, find your favourite coffee shop, read a book on the beach…
Is it worth going to Lanzarote?
Luckily, both Gran Canaria and Lanzarote are year-round destinations that are worth visiting at any time. Both get plenty of sunshine for you to be able to enjoy your stay. Both islands are relatively touristic, but you won't really see swarms of tourists in any place, on either island.
Which is prettier Gran Canaria or Lanzarote?
If you mean beach and sun, the long beaches of Gran Canaria may be what you are looking for; if, on the contrary, you prefer an island with a beach but also with many other plans to do, Lanzarote may be your ideal destination.