Update migration guide

Author: VeckoTheGecko

docs/user_guide/v4-migration.md

@@ -36,7 +36,9 @@ Version 4 of Parcels is unreleased at the moment. The information in this migrat
 ## ParticleFile
 
 - Particlefiles should be created by `ParticleFile(...)` instead of `pset.ParticleFile(...)`
-- The `name` argument in `ParticleFile` has been replaced by `store` and can now be a string, a Path or a zarr store.
+- `ParticleFile` output is now in Parquet format
+- `ParticleFile` writing behaviour now errors out if there's existing output (this be being further discussed in xxxx)
+
 
 ## Field