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I don't know of any evidence this is anywhere near true.  The CO2 increase is nearly all from burning fossil fuels.

The key is (per the graph I posted a while back), for the last million years, CO2 in the atmosphere has ebbed and flowed between 250 and 300 ppm. In the 150 years since human industrialization, it is now >400 ppm.

I have no clue where this "4%" comes from.  

Die Welt presented a common number-trick by climate deniers (readers can probably point to some english-language examples):

Quote

In fact, carbon dioxide, which is blamed for climate warming, has only a volume share of 0.04 percent in the atmosphere. And of these 0.04 percent CO₂, 95 percent come from natural sources, such as volcanoes or decomposition processes in nature. The human CO2 content in the air is thus only 0.0016 percent.

The claim “95 percent from natural sources” and the “0.0016 percent” are simply wrong (neither does the arithmetic add up – how would 5% of 0.04 be 0.0016?). These (and similar – sometimes you read 97% from natural sources) numbers have been making the rounds in climate denier circles for many years (and have repeatedly been rebutted by scientists). They present a simple mix-up of turnover and profit, in economic terms. The land ecosystems have, of course, a high turnover of carbon, but (unlike humans) do not add any net CO2 to the atmosphere. Any biomass which decomposes must first have grown – the CO₂ released during rotting was first taken from the atmosphere by photosynthesis. This is a cycle. Hey, perhaps that’s why it’s called the carbon cycle!

That is why one way to reduce emissions is the use of bioenergy, such as heating with wood (at least when it’s done in a sustainable manner – many mistakes can be made with bioenergy). Forests only increase the amount of CO₂ in the air when they are felled, burnt or die. This is immediately understood by looking at a schematic of the carbon cycle, Fig. 3.


Fig. 3 Scheme of the global carbon cycle. Values for the carbon stocks are given in Gt C (ie, billions of tonnes of carbon) (bold numbers). Values for average carbon fluxes are given in Gt C per year (normal numbers). Source: WBGU 2006 . (A similar graph can also be found at Wikipedia.) Since this graph was prepared, anthropogenic emissions and the atmospheric CO2 content have increased further, see Figs 4 and 5, but I like the simplicity of this graph.

If one takes as the total emissions a “natural” part (60 GtC from soils + 60 GtC from land plants) and the 7 GtC fossil emissions as anthropogenic part, the anthropogenic portion is about 5% (7 of 127 billion tons of carbon) as cited in the Welt article. This percentage is highly misleading, however, since it ignores that the land biosphere does not only release 120 GtC but also absorbs 122 GtC by photosynthesis, which means that net 2 GtC is removed from the atmosphere. Likewise, the ocean removes around 2 GtC. To make any sense, the net emissions by humans have to be compared with the net uptake by oceans and forests and atmosphere, not with the turnover rate of a cycle, which is an irrelevant comparison. And not just irrelevant – it becomes plain wrong when that 5% number is then misunderstood as the human contribution to the atmospheric CO2 concentration.

The natural earth system thus is by no means a source of CO2 for the atmosphere, but it is a sink! Of the 7 GtC, which we blow into the atmosphere every year, only 3 remain there. 2 are absorbed by the ocean and 2 by the forests. This means that in the atmosphere and in the land biosphere and in the ocean the amount of stored carbon is increasing. And the source of all this additional carbon is the fact that we extract loads of fossil carbon from the earth’s crust and add it to the system. That’s already clear from the fact that we add twice as much to the atmosphere as is needed to explain the full increase there – that makes it obvious that the natural Earth system cannot possibly be adding more CO2 but rather is continually removing about half of our CO2 emissions from the atmosphere.

The system was almost exactly in equilibrium before humans intervened. That is why the CO₂ concentration in the air was almost constant for several thousand years (Figure 2). This means that the land ecosystems took up 120 GtC and returned 120 GtC (the exact numbers don’t matter here, what matters is that they are the same). The increased uptake of CO₂ by forests and oceans of about 2 GtC per year each is already a result of the human emissions, which has added enormous amounts of CO₂ to the system. The ocean has started to take up net CO₂ from the atmosphere through gas exchange at the sea surface: because the CO2 concentration in the atmosphere is now higher than in the surface ocean, there is net flux of CO₂ into the sea. And because trees take up CO₂ by photosynthesis and can do this more easily if you offer them more CO₂ in the air, they have started to photosynthesize more and thus take up a bit more CO₂ than is released by decomposing old biomass. (To what extent and for how long the land biosphere will remain a carbon sink is open to debate, however: this will depend on the extent to which the global ecosystems come under stress by global warming, e.g. by increasing drought and wildfires.)