Unfortunately, that is about what it takes, at least on my '87 300E. i don't know if they changed the attachment mechanism on your 400. A better choice than a screwdriver is an 'orange stick' as sold at cosmettic counters - a flat tipped screw driver like soft wood item that won't scratch. The 'ears' of the cover may be brittle from the sun, so don't bend them too much.
2003 Firemist Red/grey leather SL 500
2015 Palladium Silver/black mbtex GLK 350
1987 Smoke Silver/burgundy mbtex 300E Sportline (SOLD)
Click to see 87 300E