  We study the vortex state in a magnetic field parallel to the $c$ axis in the
framework of the extended Ginzburg Landau equation. We find the vortex acquires
a fourfold modulation proportional to $\cos(4\phi)$ where $\phi$ is the angle
${\bf r}$ makes with the $a$-axis. This term gives rise to an attractive
interaction between two vortices when they are aligned parallel to $(1,1,0)$ or
$(1,-1,0)$. We predict the first order vortex lattice transition at
$B=H_{cr}\sim \kappa^{-1} H_{c2}(t)$ from triangular into the square lattice
tilted by $45^\circ$ from the $a$ axis. This gives the critical field $H_{cr}$
a few Tesla for YBCO and Bi2212 monocrystals at low temperatures ($T\leq 10
K$).
