The first digit is the principle aluminum alloy.
The first digit also describes the aluminum series. Ksi is ultimate tensile strength range.
1XXX

Min 99% Aluminum

10 - 27 ksi
2XXX
Copper
27 - 62 ksi
3XXX

Manganese

16 - 41 ksi
4XXX
Silicon
25 - 55 ksi
5XXX

Magnesium

18 - 51 ksi
6XXX

Magnesium and Silicon

18 - 58 ksi
7XXX

Zinc

32 - 88 ksi
8XXX
Other Elements
-
 
4043 - 5% Silicon Aluminum Welding Rods Bright Finish AWS A5.10 Class ER4043. Flows freely at 1155 F (623 C). Recommended for welding 3003, 3004, 5052, 6061, 6063, and casting alloys 43, 355, 356 and 214. Typical Composition: Si 4.5-6.0%; Cu 0.30 Max.; Fe 0.80 Max.; Mn 0.05 Max.; Zn 0.10 Max.; Mg 0.05 Max.; Ti 0.20 Max.; Others 0.15 Max.; Al Balance (95%). ALLOY 4043 has a lower melting range than the base metals for which it is commonly used and relieves contraction during weld cooling. - Used on 1xxx, 3xxx, 6xxx series alloys. 2014, 2219, 5005, 5050, 5052, 7005, 7039, N21. Al-Si, Al-Si-Mg casting alloys and combinations of above materials. Al - Lithium alloys. DTD372B, LM4, LM2, LM6, LM9, LM20, LM25, LM27.
 
4047 - 11-13% Silicon Aluminum Welding Rods Bright Finish AWS A5.10 Class ER4047. Melting range 1068 - 1080F (Eutectic point) Density: 2.66 gm/cc - Used on L159, L155, LM2, LM6, LM9, LM20. Also used for gas brazing applications. 1xxx, 3xxx, 6xxx, series alloys. 2014, 2219, 5005, 5050, 5052, 7005, 7039. Al-Si and Al-Si-Mg casting and combinations to wrought alloys as listed above.
 
5356 - 5% Magnesium Welding Rods Bright Finish AWS A5.10 Class ER5356. Flows freely at 1180 F (637 C) and can be used with the gas or gas tungsten-arc welding processes. Recommended for joining 5050. 5052. 5083, 5086, 5356, 5454, and 5456. Typical Composition: Mg 4.5-5.5%; Si 0.25 Max.: Fe 0.40 Max.; Cu 0.10 Max.; Mn 0.10 Max.; Zn 0.10 Max.; Cr 0.05-0.20%; Ti 0.06-0.20%; Others 0. 15 Max.; Al Balance (94%). 5356 is a heat treatable alloy used for welding alloys of similar composition. 5356 is increasingly used in preference to type 5056, since it contains Ti for grain refining.
 
Suggested Filler Metals for Specific Requirements
Material
Welded
Strength Ductility Color
Match
After
Anodizing
Least
Crack
Tendency
1100 4043 1100 1100 4043
2219 2319 2319 2319 2319
3003 4043 1100 1100 4043
5052 5356 5654 5356 5356
5083 5183 5356 5183 5356
5086 5356 5356 5356 5356
5454 5356 5554 5554 5356
5456 5556 5356 5556 5356
6061 5356 5356 5654 4043
6063 5356 5356 5356 4043
7005 5356 5356 5356 5356
7039 5356 5356 5356 5356