first off, the "four bars" is nonsense. it's different for everything. for example, on my laptop sat in this very position, wifi gives me five bars. on my netbook, three bars. in the exact same position.
you may have to do a factory reset, as your memory on the tablet may be taking up too much. either that, or uninstall some programs you don't use anymore to free up some ROM. another thing, close and programs you have open so you have that extra RAM space. this is pretty much all i can remember from my dad talking about wifi problems a while ago so ye