You might need to talk to a doctor if you can only go flat out for 4 seconds before having to slow down. You may put out more power during the first few seconds, but you should be able to go substantially longer than this before falling into the limitations of aerobic capacity.

I assumed this was about uphill cycling, which depending on gradient checks out to me. It's also a function of bike, cargo, and cyclist mass. Lighter cyclists find climbs easier.