I've tried this, and several other suggestions. I'm running a 64 bit os on a 64 bit machine, and I'm running 64 bit java, but the launcher still will not allow me to allocate more than 3.5 gigs. It also shows a different file than people are saying. I don't know whether if this is because I have a newer install or if it's because my install is wonky.
I'm also trying to launch an obscure modpack though for a friend's server called New Technik pack 1.7.10.
If anyone can help, I'd be much obliged. I am running this on a crappy laptop, and had issues the last time I used technic two years ago. However, I was eventually able to get that to work on one of the main packs - this time it just ain't working. I haven't checked it yet on a more prolific pack.