There are a few alternatives.... cheap drop in LED *bulbs* with 1-3 5mm LEDs in them but IMO the output of them doesn't warrant a light that size nor run time of 4D cells. Next is the everled type of bulbs there are a few brands of these that vary in price and features but essentially they all use Luxeon 1watt LEDs. Most use one 1watt with one version uses 2 of them on a board pointing out sideways. I haven't used any of these myself and various opinions arise about them, some of the complaints about them include odd color tints, brightness compared to original bulb replaced, and cost.
most of these cost between $20-40 which IMO unless you absolutely love the flashlight you are putting them in is a bad decision when you can order something like a UKE4AA eled that uses 4AA cells, runs about 10-12 hours or so, is regulated and marginally waterproof... for about $20 plus shipping. The major problem with LED *bulbs* is most lights are not able to pull the heat they generate off of them so they either have heat regulation to dim them some to keep from burning them up or are intentionally underdriven somewhat like the UKE4AA eled is. Unless a light is designed for a high wattage LED you are going to see reduced light output compared to ones designed around them made of heat sinking metals such as aluminum.
My advice is to buy an already made LED light that does what you are wanting to do here instead of spending money on an LED drop in bulb. Perhaps even find a higher output bulb for this lantern to make it brighter instead.
