The case sent waves through the country two years ago, as TV3 revealed a possible fraud scheme where plain water was delivered to Rigas Satiksme instead of procured 'nano-detergent' to clean the company's fleet.
Investigators now say these four persons had rigged the 2013 procurement for cleaning supplies, forging labels and documents. The company that won the tender, the police say, did not even have access to the cleaning supplies it would purportedly deliver.
After successfully carrying out the fraud, they repeated a similar procurement in 2014, and only stopped in early 2015 after the TV3 report came to light.
Large-scale fraud carries a jail term of up to 10 years, while one of the four persons is also suspected of money laundering, which carries a prison sentence of up to 12 years.
The police do not reveal the suspects' names, nor whether they are Rigas Satiksme employees, however earlier this year they did say that there are four people involved in the case, and all of them are from Latvia.
Following the TV3 report, Rigas Satiksme carried out an internal probe but did not find anything wrong with the tender.