jmc

There are 16 possible outcomes: $HHHH$, $HHHT$, $HHTH$, $HTHH$, $THHH$, $HHTT$, $HTHT$, $HTTH$, $THTH$, $THHT$, $TTHH$ and $HTTT$, $THTT$, $TTHT$, $TTTH$, $TTTT$. The first eleven have at least as many heads as tails. The probability is $\boxed{\frac{11}{16}}$.