I have a strange problem with samba4 and Windows 8: I can't mount iso images, it worked with samba3. Same thing for running exe files. I guess it's a permission problem though.
By the way: samba4 4.0.5 is out, which should fix the problem with jCIFS (Android) clients and unix extensions = yes.
However, it won't compile correctly:
$this->bbcode_second_pass_code('', 'Build failed: -> task failed (err #-9):
{task: manpages/smb.conf.5 smb.conf.5.xml -> smb.conf.5}
')Working around this problem by disabling man pages

Now I only have to wait till it finishes...
EDIT: Samba 4.0.5 and unix extensions = yes is working with jCIFS. Though I had to heavily modify the PKGBUILD to make it build.
In case anyone wants to have a look at my smb.conf because he's bored, feel free to do so. But I don't really need a fix at the moment, since I use Windows 8 rarely
$this->bbcode_second_pass_code('', '[global]
workgroup = WORKGROUP
server string = GOFLEX
netbios name = GOFLEX
hosts allow = 192.168.0.
printcap name = /dev/null
load printers = no
disable spoolss = yes
printing = bsd
log file = /var/log/samba/log.%m
max log size = 50
security = user
dns proxy = no
#Reject incorrect logins
map to guest = Never
#Global security
public = yes
#Performance tweaks
socket options = IPTOS_LOWDELAY TCP_NODELAY SO_KEEPALIVE
write cache size = 2097152
use sendfile = yes
getwd cache = yes
#============================ Share Definitions ==============================
[Data]
path = /media/Data
comment = Data partition
read only = no
public = yes
browseable = yes
writable = yes
guest ok = no
')