I have 4 black wires coming up to feed a spa. I have been able to determine the two hot wires but not able to tell which of the two wires is ground and which one is neutral. How can I distinguish which is which? Is it true that on a multimeter the power wire and the ground will show less power than the neutral and power?
Depends. Remarking wires is not allowed in wires smaller than #4. And we certainly don't mess around when wiring a hot tub, which can kill people.
If it's #6 or smaller wire
You splice a rope to it, and pull it all out. Then you take two of the wires to the electrical supply house and trade them for a white wire of the same size and length, and a green ground wire of appropriate size and length (if the wire is #8 or #6 then the ground wire can be #10), a competent electrical supply house will know. A big-box store like Home Depot will not.
A bare wire is allowed for ground if the wire is copper.
Then use the rope to pull the 2 blacks, white and green back into the pipe.
If it is #4 or larger
You obtain white and green tape. Big box stores sell this in a 5-pack with 3 other colors for $4.
Go back to the service panel and unhook the wire on the ground bar (which might be the same as the neutral bar if it's a main, sloppy but legal). Insulate it with plain black tape. Now go to the hot tub subpanel and attach one mystery black wire to the neutral bar, and insulate the other. If the panel powers up and all the voltages are correct, you have found neutral. Mark both ends of it liberally with white tape.
The remaining wire must be ground, mark it liberally with green tape and hook it back up.
That may seem a trite exercise if both wires land on connected or same busbars. It's not. It's vital to know which is which, if the old main panel is ever made a subpanel, or if testing ever needs to be done particularly re: the GFCI. What's more, given the critical life-safety role of hot tub wiring, any sign of unprofessional or imperfect work is something you really can't have the inspector seeing. Especialy if you have done, or plan to do, more than one of them.