Hi Rain,
When playing around with settings I noticed a 4 layer pcb is 2.5 times the cost of a 2 layer, halfing the overall price. From $87 to $44
Since you mention "4 layer for heat dissapation", I presume it may not use all four layers?
If it is the case I kindly request a two layer gerber file, or make the gerber downloadable so I can do it myself.
I reckon a copper shim on the down facing end would serve the same purpose for cheaper