Charging a lithium battery to 4.34 V
The maximum a battery can be charged is determined by the chemistry of the battery. For a lithium-polymer battery the charging curve looks like this:


Source: https://batteryuniversity.com/learn/article/charging_lithium_ion_batteries

You first give the battery a constant current determined by the cell, then a constant voltage of 4.2 volts. 


  Can be charging to 4.34 V dangerous or significantly decrease
  lifetime of the battery?


If you go to a higher voltage, it will reduce the battery lifetime or cause a failure, don't do it.

If you need a voltage high than 4.2 volts use a DC DC boost converter. 

Overcharging Lithium-ion


  Lithium-ion operates safely within the designated operating voltages;
  however, the battery becomes unstable if inadvertently charged to a
  higher than specified voltage. Prolonged charging above 4.30V on a
  Li-ion designed for 4.20V/cell will plate metallic lithium on the
  anode. The cathode material becomes an oxidizing agent, loses
  stability and produces carbon dioxide (CO2). The cell pressure rises
  and if the charge is allowed to continue, the current interrupt device
  (CID) responsible for cell safety disconnects at 1,000–1,380kPa
  (145–200psi). Should the pressure rise further, the safety membrane on
  some Li-ion bursts open at about 3,450kPa (500psi) and the cell might
  eventually vent with flame.  


Source: https://batteryuniversity.com/learn/article/charging_lithium_ion_batteries