I am making my friends wedding cake which is 2 tiers (2 x  6" for top & 2 x 9" for bottom) & need to transport it about 40 minutes away.

What's the safest way to transport it?

Any other advice?

Also is 4 dowels in the bottom tier & 1 big one in the middle enough to keep it stable?

Thanks in advance.

If you make sure they're cold when you deliver them you probably don't need a center dowel. I never use one, I deliver everything cold and nothing has ever shifted. 

Put them in a box (just a brown moving box from Staples is fine) and use a piece of non-skid carpet padding on the bottom to keep them from moving around in the box. Put some more of the non-skid pad under the box in the car (a flat area, not on the back seat obviously) to keep them from sliding around and you should be fine.

I second the cold cake transport.  But rather than use a cardboard box, I purchased a large clear plastic storage box that a bit of room at the top from Target. The box had a locking lid, and I actually turned it upside down, using the lid as the bottom.   I lined the lid with two blanket ice packs covered with a light kitchen towel and then the cake on top.  I also had ice blankets around the sides to about half way up and one on top on the outside of the box.  I then covered the whole box with a large white cotton sheet to help protect from the sun. (I was delivering a 2 tired cake 6" and  10" on a 14" cake board). Cake stayed cold and perfect despite the 106 degree heat, a car without air conditioning, and inadvertently having to take a very bumpy dirt road, due to GPS being off.  

On a side note I hit a local auction last year, where I purchase Ice Cream vendor bags that are usually used for the walk around vendor ice cream sellers. These actually came specifically from Candlestick park.   I lucked out and got 8 of them in two sizes for about 5 bucks each.  Anytime I am delivering cupcakes/small cakes/small items that need to stay cool, I use the bags. They work great! In addition to deliveries, I use them to keep my cakes/cupcakes cool at shows and farmers markets.  With those, I add ice blankets which extends them to lasting 4 to 5 hours in well over 100 degree heat.  It is possible to buy the bags new for a bit more, but knowing what I know now, I would find them an excellent investment.