First, current 2nd generation Intel Core processors does have the headroom to encode 4 1080p streams concurrently (in HW) at >=30fps.
Regarding method for combining the streams into a wall. There are many options depending on your setup:
1) Assuming you want the 4 streams fit into a D1 size frame then for NTSC that would mean that each stream needs to be re-sized to 360x240, right? If so, than first re-size each raw 1080p stream using either Media SDK VPP or DirectX. Then combine the 4 raw 360x240 into one D1 frame that is used as input to Media SDK encoder. The frame combining operation is out of scope of Media SDK but it could instead be done manually or probably most efficiently using D3D surfaces with DirectX StretchRect(). In that way resizing and combining can be performed in the same operation.
2) Encode the 4 streams separately andsendthem to the client residing oniPad. TheiPadclient then decodes the 4 streams andrenders them as a wall appropriately. This approach will potentially put more load on theiPadclient since 4 streams have to be decoded concurrently. Also, if the streams originate from different machines, then this solution is probably the one you must use.
I'm not sure I have grasped your exact use case here so forgive me if I'm not hitting the mark.