Yes, I agree that you should be in doubt about updating to a GTX 980. The card is not much different than the 970, and it is a lot more expensive.

GTX 970 vs. GTX 980:

Cores: 1664 vs. 2048
Base Clock: 1050Mhz vs. 1126Mhz
Texture Fill Rate: 109GT/s vs. 144GT/s
VRAM: 3.5GB vs. 4GB

EDIT:
And to answer your questions:

Q: Will the 980 behave just like the 970 (only that the 980 will have issues at 4GB instead of at 3.5GB)?

A: Yes. However, it does have a few more CUDA cores. This will result in a greater overall performance.

Q: Will the 980 behave just like the 970 (only that the 980 will have issues at 4GB instead of at 3.5GB)?

A: I suppose so. Most games will try to regulate the amount of VRAM they use in accordance to how much you have. The reason the 3.5GB is an issue on the 970 is the fact that games think they can use all 4GB, and when they dip into that 'defective' 0.5GB, everything slows down.

Q: Will performance degrade in a more progressive way (slowing progressively, or just losing details from the textures)?

A: I don't really understand the question. If you play at 4k and use high res textures, the game will start lagging and your fps will go down, because they game does not have enough VRAM to hold all the information.

Yes, I agree that you should be in doubt about updating to a GTX 980. The card is not much different than the 970, and it is a lot more expensive.

GTX 970 vs. GTX 980:

Cores: 1664 vs. 2048
Base Clock: 1050Mhz vs. 1126Mhz
Texture Fill Rate: 109GT/s vs. 144GT/s
VRAM: 3.5GB vs. 4GB

EDIT:
And to answer your questions:

Q: Will the 980 behave just like the 970 (only that the 980 will have issues at 4GB instead of at 3.5GB)?

A: Yes. However, it does have a few more CUDA cores. This will result in a greater overall performance.

Q: Will the game just crash if it tries to use more than 4GB of VRAM?

A: I suppose so. Most games will try to regulate the amount of VRAM they use in accordance to how much you have. The reason the 3.5GB is an issue on the 970 is the fact that games think they can use all 4GB, and when they dip into that 'defective' 0.5GB, everything slows down.

Q: Will performance degrade in a more progressive way (slowing progressively, or just losing details from the textures)?

A: I don't really understand the question. If you play at 4k and use high res textures, the game will start lagging and your fps will go down, because they game does not have enough VRAM to hold all the information.

Yes, I agree that you should be in doubt about updating to a GTX 980. The card is not much different than the 970, and it is a lot more expensive.

GTX 970 vs. GTX 980:

Cores: 1664 vs. 2048
Base Clock: 1050Mhz vs. 1126Mhz
Texture Fill Rate: 109GT/s vs. 144GT/s
VRAM: 3.5GB vs. 4GB

Yes, I agree that you should be in doubt about updating to a GTX 980. The card is not much different than the 970, and it is a lot more expensive.

GTX 970 vs. GTX 980:

Cores: 1664 vs. 2048
Base Clock: 1050Mhz vs. 1126Mhz
Texture Fill Rate: 109GT/s vs. 144GT/s
VRAM: 3.5GB vs. 4GB

EDIT:
And to answer your questions:

Q: Will the 980 behave just like the 970 (only that the 980 will have issues at 4GB instead of at 3.5GB)?

A: Yes. However, it does have a few more CUDA cores. This will result in a greater overall performance.

Q: Will the 980 behave just like the 970 (only that the 980 will have issues at 4GB instead of at 3.5GB)?

A: I suppose so. Most games will try to regulate the amount of VRAM they use in accordance to how much you have. The reason the 3.5GB is an issue on the 970 is the fact that games think they can use all 4GB, and when they dip into that 'defective' 0.5GB, everything slows down.

Q: Will performance degrade in a more progressive way (slowing progressively, or just losing details from the textures)?

A: I don't really understand the question. If you play at 4k and use high res textures, the game will start lagging and your fps will go down, because they game does not have enough VRAM to hold all the information.