If you do not wish to wait further, then you may go for the BS4 variant. Moreover, BS6 compliant cars are likely to get a price hike once in the market. Also, buying a BS4 compliant car would not affect your owning experience even when the upcoming norm kicks in.
Buying a diesel car now won\'t make any difference in future after the launch of BS6 compliant engine as the availability for spares and service will be there after sale and as of now there is no official update regarding the discontinuation of the diesel engine. So, if you are in a need of a car now you can go with the current BS4 norm car and if you are looking for the better emmision norms and updated car you can wait for the launch of BS compliant car.