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I've read that when you attach more than 3 gpu's to one motherboard without using powered risers that can actually damage the mobo.
I've also noticed a plug on mobo that I am going to attach 4 gpu's on.
Does plugging in 12v into this plug would solve the possibility of frying mobo without using powered risers?
It looks like it is designed for that purpose. It's also tagged pci_e 12v.
http://i41.tinypic.com/1pgjdy.jpg

Well I've no experience with that particular board, but if it is indeed a molex port designed to feed pci e then it should help yes. The manufacturer site is very unclear about what it is exactly though, so definitely be cautious (feel your cables for signs of heating)

These have been around for a while. Essentially its a cheaper way for them to power cards by reducing infrastructure within the rest of the board.

From previous experience it will not help at all.

Back before BitCoin mining was cool, I have run a few quad crossfire setups. The first one burned because I had a single fan running off that molex port (obviously the PSU was connected to it too) ... and the small amount it drained was enough to tip the scales and the main socket burnt.

You're right that 4pin molex connect is there to provide additional power to the pcie slots.  Each pcie slot will/can provide 75 watts to the card.  Really to answer your question, we need to know what type of cards you are running.  If you're planning to run four 7770s or 7790s, then you'd be fine as each of these cards also has a 6pin pcie power connect that will also provide 75 watts, thus not making the mobo provide the full 75 watts out of its pcie slots.  These cards (7770/7790) consume 80/85 watts respectively, but will have access to 150 watts via the pcie slot and their pcie 6pin power connector.

If you're running cards like 79xx that consume more watts, you need to get powered risers.

I have come across this powered risers offer

http://www.dhgate.com/100pcs-lot-pcie-express-x16-riser-card-flexible/p-ff8080813de859d9013e36a84b397cb1.html

is dhgate.com legit, anyone had experience dealing with them?

I can get them for cheaper than that price if you are ordering in 100+ pcs

How could you possibly be running a fan off that molex port?
The molex port provides power INTO the motherboard, not out.

Those molex slots help a lot in my experience. Got one running 5x GPUs without the need of a powered riser. Been rock solid for 2 yrs.
On the other hand, another board with 4 cards burned up within weeks.

You are stupid. You aren't supposed to plug in peripherals into that molex port. You are supposed to plug in molex power from your power supply. You broke your own motherboard.

It's believed that the GPU Core (and sometimes VRAM) pulls power from the PCI-E power plugs. Other small things on the PCB such as the fan, minor logic stuff, etc (and sometimes VRAM) pull power from the PCI-E slot, which pulls power from the ATX connector, which only has 2 12 volt wires. If you pull too much power, you will burn out those pins on the ATX connector. Its better to have a board that natively has a molex or SATA connector for supplementary power (Adds 1 12v and 2 grounds), and best to have a board that uses a PCI-E plug for supplementary power (adds 3 12v and 3 grounds). If you don't have that on your motherboard, using powered risers is a cheap fix.

http://www.btc-poolwatch.com/public/images/mobo_atx.jpg

Some cards had designs where components on the boards pulled exclusively from the PCIe slot. I believe it was the HD 5870 that pulled the current to power the GDDR from the PCIe slot which resulted in a lot of overclockers frying the 24-pin connectors on their boards when you were running memory intensive benchmarks with a quadfire setup. I would like to believe that AMD/NVIDIA have fixed this but I wouldn't place any bets on it. They are allotted 75w(or is it 50w) from the PCIe slot and they are going to use it.