The BMD-4M is between the new 25 ton and the 10 ton vehicle families... closer to the 10 ton than the 25 ton.
The fact that they want heavier armour suggests to me they are going to do what they did before.
The BMD is basically a BMP heavily modified for air dropping and specialist use.
As said above the Kurganets-25 and presumably the Boomerang-25 will be too heavy for air dropping, which rules them out, while the Boomerang-10 is probably too light... and it is likely they will want tracked vehicles for mobility for many uses anyway.
For a very small batch I would think that actually making BMD-4Ms now that in 2016 can be fitted with the electronic suites of the Kurganets-25 family vehicles makes sense... the VDV has a separate logistics support train so the fact that it is not the same as the Armies vehicle wont actually matter that much as long as it is consistent within the VDV.
They have been doing what the Army plans to do for quite some time except instead of using the new vehicle families being developed they are going to use the BMD-4M as the basis, so they will have a tank (Sprut) and an IFV (BMD-4M), a troop carrier (BTR-MDM), artillery (2S9M Nona), recon vehicles EW vehicles command vehicles etc etc.
This means the force consists of the same basic vehicle with the same engines and the same transmissions/gearboxes/components etc etc so they use the same fuels and lubricants and spare parts and tools.
Yes, I know I am flip flopping, but the idea of saving money by buying and upgrading BMD-2s till something new is ready only makes sense if they are planning to use a new vehicle being developed.
If they plan to use an upgraded BMD-4M then it makes the most sense to simplify the current design to minimise cost... perhaps including modular add on armour made of expensive and exotic materials that is not fitted initially to reduce costs and simplified systems for now, and then in 2016 when a full upgrade package is ready you can start applying that to in service vehicles and producing new vehicles to match the new standard.
It means you can start training for night and all weather and net centric combat with new sighting and navigation and battle management systems without spending too much now and then in 2016 when the new stuff is ready there will be more money in the budget to buy the new stuff when it is ready.