Itinerary question

The train route from Barcelona to Granada doesn't go through Valencia; The express rail route runs through Zaragoza, Madrid and Cordoba. There are parts of the coast south of Valencia where there are no trains, not even slow trains. If Valencia is very important to you, you can take the train from Barcelona to Valencia and then from Valencia to Madrid, but that will take several hours longer that a direct Barcelona-Zaragoza-Madrid train.

Although Valencia is a nice city, I would rank it way, way below Seville and Cordoba, which are not on your current itinerary. That part of Spain is miserably hot in the summer, so April is pretty much an ideal time to be there. Things will be busy (and hotels more expensive) in Andalucia during Holy Week, which is April 13-19 next year. Seville, in particular, is an interesting experience during that period, with many processions. Most sights remain open, but there can be adjustments in their opening hours. I'd decide whether the cultural experience of Holy Week is more important than somewhat more efficient sightseeing and lower lodging costs and adjust my itinerary accordingly. If you are going to be in Andalucia during Holy Week, you need to pin down hotel reservations ASAP.

I would try to avoid splitting my nights in Madrid, because checking in and out of hotels twice in the same city is inefficient. Your time allotment for Madrid (4 nights) is appropriate for folks interested in hitting a bunch of the city's excellent art museums. If you are not big art lovers, you might consider trimming the Madrid time a bit.

I'm a huge fan of Barcelona. A lot of folks might be satisfied with just 4 nights there; some even try to do the city in 3 nights. Modernista architecture junkies like me want to stay longer. Barcelona isn't cheap, and the major sights are usually very crowded. If you haven't already done so, give some thought to how you plan to spend your time there and whether your sightseeing plans will have you negotiating mobs at most of your stops. The biggest issues are usually at these sights, all of which require timed tickets that should be purchased in advance, because the tickets lines can be massive: La Sagrada Familia, Parc Guell, Casa Mila/La Pedrera, Casa Batllo and the Picasso Museum. Barcelona has many other sights that not so overwhelmed, but it seems most of those five are on almost every tourist's list.