What's powering the speakers? Typically you'd already have a receiver (or preamp or integrated amp) with an input-selection switch.*
And, hopefully you have an amp for each room, or a 70.7V constant-voltage system? (Connecting a bunch of speakers in parallel lowers the impedance and that can damage your amplifier, and you need separate amps if each room can select different audio sources..)
For the inputs can I use 4 relays?
Yes. A double-pole double throw relay for each input. Or, you could also use a digitally controlled Analog Switches
. (If you are going to build this thing, I'd add a couple extra inputs for unknown future requirements.
how can I reduce and increase volume?
A Digital Pots
, assuming you are controlling line-level signals (not the speaker outputs). A digital pot won't "increase" the volume, but you shouldn't an additional gain stage.
Im absolutely new to hardware, and want to get started
I suggest you tackle one part at a time. I have a feeling the IR remote is the most difficult part and I'd start by turning on & off an LED (and then maybe dimming the LED).
You can also build & test the relay switching separately, and the digital-pot volume control separately. There are at least 3 different kinds of digital pot. I've never used a digital pot, but I believe there is a type that you simply feed-in an up-down pulse and that seems like the simplest to interface & "program".
* It's OK to connect more than one input
together.. So for example, you can use Y-Adapters (splitters) to connect the CD player output into 4 amplifiers/receivers. However, it's NOT OK to connect multiple outputs
together, so you can't connect the CD-player output to the radio output. For that you need relays to select one output at a time, or a mixer to mix the audio signals.