Are Rock Band 4 instruments backwards compatible with Rock Band 1, 2, or 3?

From what I've been seeing, it doesn't seem like the Rock Band 4 instruments are backwards compatible with Rock Band 1, 2, or 3. According to posts in this thread:

Rock Band 4 instruments only work with Rock Band 4. If you want to play an older game like 2 or 3, you're going to need instruments that were built specifically for those family of games on that particular console.

I can affirm this, as I've attempted to use my Rock Band 4 Stratocaster guitar to play Rock Band 1 and 2 on my PS3, but it wouldn't connect. I don't own Rock Band 3, however.

It's important to mention that it looks like the Rock Band 3 Stratocaster guitar (not the pro Mustang guitar), drum set, and microphone are compatible with Rock Band 4, assuming you didn't switch console families, so you wouldn't need to buy an entirely new set of instruments, assuming you're currently using the default Rock Band 3 instruments. As an aside, the microphones are completely compatible across Rock Band 1-4, as they are simple USB plugins.

If you are playing on XBox One, you will need an adapter for instruments designed for the XBox 360. If you are moving from PS3 to PS4, the dongle for the PS3 will work on the PS4.

For a complete breakdown of what instruments are compatible with Rock Band 4, see this official instrument compatibility chart.