Available four chars domains with results in the WayBack Machine

As always, all of them were available at time of posting.
Note the presence of some LLLL.net :-)

6KDS.COM
EP-R.COM
DNCF.NET
F2BP.COM
M602.COM
RFLA.NET
VNSF.NET
R500.NET
NL4H.COM
NP75.COM
GIDG.NET
MDRY.NET