I plan to power four 240v outlets from a single dual pole 100A breaker using a power distribution block. I'll have 4 AWG copper wires coming from each terminal feeding into a power distribution block, and will have separate 10 AWG copper wires connecting the block to each outlet (two are rated at 30a the others at 20A). Is this safe and legal? My research tells me I should be alright, but I'd rather be safe than sorry. The outlets will be powering computers rated at 24A for the 30A outlets and 16A for the 20A outlets.
Only if the ”distribution block” is a subpanel
Just get an 8-space main-lug panel and four 2-pole breakers.
"Just". As if electrical is that simple. I know you did a lot of research, but it's a broad subject.
You didn't say whether you needed neutral for 120V loads. 240V PCs tend to not need it, so I omitted it from the drawing. It's not a big deal, just land it on the neutral bar, which is not breakered. Neutral is not ground, and ground is not Vss. That green thing is equipment safety earthing, which we call ground. Note that in a subpanel, neutral bar and ground bar are separated.
If you use a subpanel that doesn't come with separate ground bars, and you reuse its neutral bar for a ground bar, mark it clearly as such, so the next guy doesn't think it's neutral.
Here, note I did a shabby job of showing separation between neutral and ground bars.
I don't know whether you're running 60A or 100A, you seem unclear yourself (uh-oh!) #4 wire is wrong for both. For 60A, use #6 copper wire.
For 100A, you'll need #3 copper wire, or preferably #1 aluminum wire from the main panel to the sub. The ground can be a smaller size as Code specifies.
Your wire to the 30A receptacles needs to be #10 (larger is always OK).
Your wire to the 20A receptacles needs to be #12.
If you're running in conduit, white wire can only be neutral. Neutral must be the same size as the hots. The two hot legs don't need to be black and red - black and black will suffice, there's no need to tell them apart.
If you're using multiconductor cable the ground will be sized correctly. If you don't need neutral, use /2 cable (black white ground) and tag the white wire with tape to indicate you are using it as a hot. If you need neutral, use /3 cable (black red white ground).
Remember to derate the circuit, loading it to only 80% of capacity. You have done that with 24A PC on a 30A breaker etc.
Because breakers protect wires and devices. A 100A breaker will fail to protect #10 wire, and the wire will set your house on fire before the breaker knows to trip. It will also fail to protect a 30A receptacle if that has internal problems. If the PC it's plugged into has an internal fault that flows 70A, it will also fail to protect the PC.
It is also illegal to fit a 30A receptacle on any breaker except 30A (for this very reason). Likewise, a 20A receptacle can only be protected by a 20A breaker. National Electrical Code, Table 210.21b3.