Yes, it is safe to charge *any* NiMH cell in a D4, but you are going to have to charge for a long time as you only have 750mAh max charge rate... so 10 hours of charging and you have only charged it roughly half ways to full as there are some charging losses. Figure 24hrs to fully charge a D cell and it should be close.
Now, those D cells won't fit in the charger, but I have confirmed that adapters I have will fit my D4, but not that well.... They do fit my C9000 charger perfectly and let me charge C and D cells in my AA and AAA charger.
If you search Ebay for '2-Pack C & D Cell to AA Battery Adapter BT-C2000 BT-C2400 MH-C9000 BT-C3400' you can see the type of adapter I'm using. I have no idea if the price is good or if there's better to be had, but at least you'll see a picture of what you are looking for. A C cell and an AA cell is the same length, so I popped in an AA cell into the adapter and put the adapter in the D4 charger and it works... just don't move it around too much as it's not solid, but it will do for what you want.
Personally, I'd spend a few more $$ and get myself an Xtar SV2 which will not only charge D cells, but also C and AA and AAA, not to mention your small and large LiIon cells, as charge rates go from .250mAh to 2Ah per slot. It won't do LifePO4 cells, nor the newer 4.35V Lithiums (actually, it will charge them, but not fully.... same as your D4), but if you don't plan on either of those cells... the SV2 is perfect.
Hope that helps,
Christian
