NPN-switching 4V with a 3.3V signal
Since you are expecting to run some larger currents a two stage driver would be better as shown below.

Note Q2 needs to be able to handle the current you require.







simulate this circuit &ndash; Schematic created using CircuitLab

Since you are only using a 4V supply with an 8R load, Vce_sat becomes a major proportion of the output, as such a low Rds_ON P-MOSFET may be a better choice for Q2.