四 (sì) is the character for the number four.
The most common use for 四 on the Chinese menu is perhaps as 四川 (Sìchuān), as in Sichuan food (四川菜/Sìchuān cài) — 四川 literally means "four rivers". 四川 is sometimes abbreviated simply to 川 — note that 四 on its own never means "Sichuan", though.
The next most common is probably as 四季豆 (sì jì dòu), or green beans. The literal translation is "four season beans" — I've read that this is a reference to their being in season all year round.
As noted on Monday, the pronunciation of 四/sì is a little tricky. This YouTube video of a woman cooking 涼拌四季豆 (liáng bàn sì jì dòu, or green bean salad) helps demonstrate the proper pronunciation, since she says 四季豆 quite often towards the start of it!
The radical of 四 is 囗 (wéi/enclosure). At first glance, this looks fairly similar to 口 (kǒu/mouth), which is also a radical. The way to distinguish them is this: when the radical is wéi, the character is basically a box with something in; but when the radical is kǒu, the character will contain one or more empty boxes somewhere within it.
A few examples:
|味||wèi||(kǒu radical)||flavour/taste — used on menus as e.g. 怪味兔 (guài wèi tù), or "strange-flavour rabbit", a Sichuan dish.|
|品||pǐn||(kǒu radical)||product/commodity — often used on menus to indicate the dessert section, as 甜品 (tián pǐn), literally "sweet things".|
|回||huí||(wéi radical)||revolve/rotate/return — used on menus as 回鍋肉 (huí guō ròu), or twice-cooked pork.|
|四:||sì||radical 31 (囗)||Cantodict||MandarinTools||YellowBridge||Zhongwen|