It's impossible to say definitively whether or not Iran is closer to building a nuclear bomb now than it was four years ago. It's important to remember that fear of a nuclear Iran crops up every few years and it's never ended in nuclear fallout.
The most recent worries began to ratchet up in November 2011, when the International Atomic Energy Agency released a report saying it had "deep and increasing concerns" over the military dimension of Iran's nuclear program.
After the report, the Obama administration began enforcing a series of international sanctions to put pressure on Iran's economy. Obama believes Iran would willfully give up it's ambitions for a nuclear weapon to save its economy. Iran has always maintained that it's nuclear program is for peaceful purposes only.
The Iran issue has caused tensions with Israel, which seeks backing from the United States for a potential pre-emptive strike on Iranian nuclear installations. Obama, however, has refused to support any strike or reveal his "red line," the point at which military action is necessary.
Romney also supports tough sanctions but has used stronger language, similar to the language used by Israeli Prime Minister Benjamin Netanyahu. Romney says he would not tolerate Iran having " nuclear weapons capability." Obama insists he would not tolerate Iran "possessing" a nuclear weapon. This difference suggests Romney would support an attack on Iran more quickly than Obama.
Really, the whole Iran issue is mostly about Israel. Netanyahu wants US backing for a preemptive strike on Iran. Obama won't give it. Romney would.