The 62-year-old German leader also defended her fiscal policy after Social Democratic challenger Martin Schulz pledged to increase public investment.
"We now can not spend the money that we have", Merkel argued, pointing to planning and capacity bottlenecks in the construction industry as well as at the level of regional authorities. A full fourth term would extend her chancellorship to 16 years through 2021.
Such an investment push would cost "a lot of money", Schulz said but it would be better to spend the country's budget surplus that way than wasting it by promising voters tax cuts - a clear dig at Merkel's conservatives. "I don't see the main problem as the question of whether there should be more money - you can and you must do that - but rather in accelerating planning".
German Chancellor Angela Merkel refused to place an upper limit on refugees that the country accepts, speaking in an annual interview broadcast on Sunday.
Merkel reprised the spending theme in front of a business group on Monday, telling them in the Baltic port city of Stralsund, home to her constituency, that she'll press ahead with her trade agenda and seek full employment in Germany by 2025.
Many people in the past had taken the European Union and its advantages for granted - such as freedom of speech, freedom of religion and freedom to travel, said Merkel who grew up in communist East Germany.
Germany's current-account surplus has been on the receiving end of persistent criticism, including most recently from the International Monetary Fund and the administration of U.S. President Donald Trump.
Germany has been criticized internationally for a lack of domestic spending.
Merkel will likely be reelected in September elections opinion polls show.
SPD leader Martin Schulz has accused Merkel of making empty promises about Germany's economic and political future as the former president of the European Parliament set out his own plans to boost investment and enhance European unity.