The Saddlers say the fee for Henry, 19, could rise to more than £5m if additional clauses are met.
The left-back, currently sidelined with a shoulder injury which may require surgery, has agreed a five-year deal to rejoin his former boss Dean Smith.
Kaikai, 21, moves to Griffin Park on a deal until the end of the season.
The wide man, who scored 12 goals in 26 League One appearances during a loan spell at Shrewsbury last season, made his Premier League debut for the Eagles in May.
Meanwhile, Brentford midfielder Ryan Woods has extended his contract until 2020 and Emmanuel Ledesma has joined Greek Superleague side Panetolikos on a free transfer.
The 28-year-old Argentine made two appearances for the Bees after signing non-contract terms with the west London club earlier in August.
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Championship side Brentford have signed defender Rico Henry from Walsall for a fee of £1.5m and brought in Crystal Palace winger Sullay Kaikai on loan.