diff --git a/sf/exp4/user_prior.py b/sf/exp4/user_prior.py
new file mode 100644
index 0000000..b730263
--- /dev/null
+++ b/sf/exp4/user_prior.py
@@ -0,0 +1,26 @@
+def user_defined_lnprior(p):
+    '''
+    Takes a vector of stellar parameters and returns the ln prior.
+    '''
+    if not ( (p[2] < 0.5) and (p[2] > 0) and
+             (p[3] < 1000.0) and (p[3] > -1000.0) and
+             (p[4] < 500.0) and (p[4] > 2.0) and
+             (p[6] < 1500.0) and (p[6] > 1000.0) and
+             (p[11] > 0.0) and (p[11] < 2.0) and
+             (p[12] < -1.0) and (p[12] > -2.0) and
+             (p[13] > 0.0) and (p[13] < 300 ) ):
+
+    	return -np.inf
+    
+    # Solar metalicity to within +/- 0.05 dex
+    lnp_FeH = -1.0*(p[2] - 0.01)**2/(2.0*0.001**2)
+    lnp_vz = -1.0*(p[3] - 110.0)**2/(2.0*20.0**2)
+    lnp_vsini = -1.0*(p[4] - 105.1)**2/(2.0*30.0**2)
+    lnp_l = -1.0*(p[13] - 124.4)**2/(2.0*10.0**2)
+    lnp_logAmp = -1.0*(p[12] + 1.40)**2/(2.0*0.03**2)
+    lnp_T_BB = -1.0*(p[6] - 1100.0)**2/(2.0*10.0**2)
+    # No prior on v_z.
+    ln_prior_out = lnp_FeH + lnp_vz + lnp_vsini + lnp_l + lnp_logAmp + lnp_T_BB
+
+    return ln_prior_out
+