You would say **less than** or **the lesser of**.  Not **lesser than**.

However, it largely depends on the sentence in which you're using your particular example, as it may be that using '**fewer than**' instead of '**less than**' is correct.

- 'Less' means not as much
- 'Fewer' means 'not as many'


For example, if I'm holding three apples I have '*fewer than 4 apples*'.  

If I'm holding half a kilogram of sugar, I have '*less than a kilogram of sugar*'.

Here's a BBC article highlighting this often-made mistake:
http://news.bbc.co.uk/1/hi/magazine/7591905.stm