how much power

Tls49,
If the amp is rated for 6Ohms minimum, than it will not have enough current for 4 Ohm speaker. Not only the output power is jeopardised because of mismatched impedance and lack of current, but also the remaining power is jeopardised by heat.
If OP gets half of rated power to the speaker, consider that as a best case scenario where the worst is fried amp even on moderate 75dB volumes.

Conserning Onemug's statement in general it's true that 50W and 100W will have a slight difference in SPL but in this particular case it's a substantial difference.

So Roxito,
You can have amp as low as 50Wpc, but it should be rated stable for 4Ohm load or preferably lower.

You just need better amp, not necessarily more powerful but probably higher current with more sophisticated power supply. Vienna are good speakers and deserve good amplification.

Onemug,
Impedance being fairly flat is only one key to Maggies being an 'easy' load.
Second is line source. They couple very 'efficiently' into a room and don't drop level as rapidly as 'box' speakers of conventional design. The difference is not subtle
Third is phase. They don't have any of the wacky reactive behavior of some speakers which are at higher sensitivity.

Than again. some amps while measuring well into a resistor, do poorly with some types of reactive load.....be that capacitive or inductive.

To properly characterize a speaker as good/bad from the electrical load aspect, you need the phase data as well. Look up ANY of the stereophile speaker tests and they'll have such data. Any speaker with a big dip in impedance AND a high phase angle will be 'red flagged'. The warning will include advice to 'get thee a good amp'.

What is the minimum amout of current an amp has to deliver, to be considered a 'high current' amp?

Rok2id, the amount of specified current isn't important. A stability to the certain load is. An amplifier that delivers 60W onto 4Ohms presumed to have sufficient current to drive 4Ohm speakers.