From the help link from the page of negotiating with any FA:
"In a two-year contract, neither season can comprise more than 60% of the total value of the contract. In a three-year contract, no season can comprise more than 40% of the total value of the contract. In a four-year contract, no season can comprise more than 30% of the total value of the contract. In a five-year contract, no season can comprise more than 25% or less than 10% of the total value of the contract."
Also true, but not in the help: (1) The bonus doesn't count, so you can use a bonus to front-load a contract, and (2) for a 4-year deal no single season may be worth less than 1/6 of the total value (I worked this out by trial and error).