Formula for using data from 4 fields and total it in the 5th field

Thanks for the response. I tried this formula but the problem is the data in Field 1, Field 2, Field 3, Field 4 are in decimals ex 23.89, 21.31, 44.89, 81.98. When I tried this formula
field('Field 1') + field('Field 2') + field('Field 3') + field ('Field 4') to total in 'Field 5',
I get this below error

error: no implicit conversion of BigDecimal into String

Could you please help?