The alternator doesn't put out power by itself - it has to have power 'drawn' from it by accessories. So it won't put out 150A unless you have accessories pulling that much from it. As someone said, be sure as many accessories are off as you can - including that fan which pulls a bunch of amps. As long as you do that, you should be fine starting it up and even for short drives. Just watch your temps. But get the 4 gauge on it asap. If you have a mini-starter, you can simply run the 4 gauge down to the large gauge hot cable on the starter with your fusible link in between. Save you some cable. That approach will not work if you have the older style large single-wire starter; you'll have to run the alternator cable over to the battery or the hot side of the fender mounted starter switch.