Among the improvements to be utilized within QEMU 1.4.0 include Linux VFIO support for the PCI Express extended configuration space, CPU usage and throughput improvements for USB 2.0 devices, usb-tablet is now supported as an USB 2.0 device, improved support for pass-through of USB serial devices, a new usb-bot device that is like usb-storage but configured like other SCSI adapters, and virtio-net supports the multi-queue operation.
When it comes to the hardware support within QEMU 1.4, the MIPS architecture support has improved support for DSP instructions, PowerPC supports NVRAM, MSI support for the e500 adapter on PowerPC, improved S390 emulation, fixes to Intel Q35 chipset emulation (includes experimental AHCI migration support), and various other architecture/hardware improvements.
QEMU 1.4 also integrates various Xen improvements, one KVM improvement, the block device layer supports TRIM/DISCARD operations on block devices and on file-systems besides XFS while being fully asynchronous, block device mirroring support has been greatly improved, and there's a new experimental threaded back-end for virtio-block-pci.
There's a lot of exciting features for QEMU 1.4 considering the short time since QEMU 1.3. For more information on the new release, visit QEMU.org.