I have a 150amp panel in my home, which is decently full, but probably
has space for about another 70amps worth of breakers, but only about 4
spots left.

I HAD planned on running a 70 amp breaker, and then 4 gauge wire to my
garage to a new breaker panel. That will probably max out my panel in
my house. I've done all of the math, and I'm pretty sure it can handle
that.

One of the reasons I am doing this is to run some 230V tools in my
garage. Now recently, I may have come across the opportunity to get a
used cheap Millermatic 250 Welder, which is going to pull 44-45 amps vs
the original 20 amps, I was going to pull with a smaller Welder. This
welder all by itself would require a 60 or 70 amp breaker. So now, I
would like to run 100 amps to my garage instead.

Because of this, I'm thinking I need to increase my House Panel from
150amps to 200amps. What all is involved in this? Does the electrical
company usually do this for free? I know I will need to buy a bigger
main breaker, will there be anymore cost involved?

Is 4 gauge NM-B still ok for 100 amps, or is it pushing it too much?
Pushing that much through it, does that present a fire risk, or is 4
still enough? That kind of distance should be a 2% or less drop, which
is ok, just curious about heat.

This will be through my Basement Ceiling Joists, then up a Garage
finished wall, then through the Garage Attic, then to a Breaker Panel.
Should be about 70-80 feet.

Thanks, James