It's been far too long since I did any real electronics, so figured it was better to ask than not.

I have 3 circuits that I want to power, all take 5V but draw different amounts of current.

Now, without a reason not to, I'd power them all off the same +5v line.

Such that the 5V 4A supply is a regulated power brick, and the switch turns all on/off at the same time. The Amps are maximum draw, not constant.

It strikes me as just too simple, and something keeps trying to tell me that I need to do more.

So, is this the right way? or should I be doing something else, like sticking other components in somewhere?

 

What you are doing is OK. Your total current requirement as shown is 3.5A.
Your supply ought to deliver 4A but this could be pushing its limits.
If the total draw of 3.5A is the peak and the average current draw is much less than 4A then this is ok.

 

The 2A part is a USB hub, so its only 2A if all 4 ports are drawing 500ma each.

In reality, its far more likely to be 2-300mA than 2A.

So, yea, most likely well under 3.5A in total normal draw, 3.5A would be absolute peak.

If the PSU doesnt deliver as required, I can always just get a bigger one.

Guess its soldering time then!

 

What you are doing is OK.

+1
Just to add a vote that this is fine. The better the power supply, the more fine it is. Any potential problem stems from the supply not being able to maintain itself under load.

There could be additional issues if one of your loads was inductive, like a big solenoid or something, or was oscillating its current draw enough to disrupt the others. Probably not a concern.