will i miss vital story?
No. Valkyria Chronicles 2 was kind of a sequel to the 1st game & 3 was a side story to the first game. However, the description SEGA put out for 4 seems to indicate that this will take place in a completely different location. There might be references to the stuff going on in Galia during the war, but this seems very much to be its own thing.
Probably the best comparison I could make is Ogre Battle 64. Sure, knowing what happened in the Zenobian War in the 1st game will give you context for certain characters, but it's very much its own story.
That being said, I highly recommend playing the 1st game if you have the opportunity & the hardware. It's like $13 right now on PSN for the PS4 version, and it's one of the best games ever made.