Update v4-migration.md

Author: erikvansebille

docs/user_guide/v4-migration.md

@@ -10,7 +10,6 @@ Version 4 of Parcels is unreleased at the moment. The information in this migrat
 - `particle.delete()` is no longer valid. Instead, use `particle.state = StatusCode.Delete`.
 - Sharing state between kernels must be done via the particle data (as the kernels are not combined under the hood anymore).
 - `particl_dlon`, `particle_dlat` etc have been renamed to `particle.dlon` and `particle.dlat`.
-- `particle.dt` is a np.timedelta64 object; be careful when multiplying `particle.dt` with a velocity, as its value may be cast to nanoseconds.
 - The `time` argument in the Kernel signature has been removed in the Kernel API, so can't be used. Use `particle.time` instead.
 - The `particle` argument in the Kernel signature has been renamed to `particles`.
 - `math` functions should be replaced with array compatible equivalents (e.g., `math.sin` -> `np.sin`). Instead of `ParcelsRandom` you should use numpy's random functions.
@@ -27,7 +26,7 @@ Version 4 of Parcels is unreleased at the moment. The information in this migrat
 ## ParticleSet
 
 - `repeatdt` and `lonlatdepth_dtype` have been removed from the ParticleSet.
-- ParticleSet.execute() expects `numpy.datetime64`/`numpy.timedelta.64` for `runtime`, `endtime` and `dt`.
+- ParticleSet.execute() expects `numpy.datetime64`/`numpy.timedelta.64` for `endtime`. While floats are supported for `runtime` and `dt`, using `numpy.datetime64`/`numpy.timedelta.64` for these arguments too is encouraged.
 - `ParticleSet.from_field()`, `ParticleSet.from_line()`, `ParticleSet.from_list()` have been removed.
 
 ## ParticleFile