According to Mr.Eclipse,
The luminous transmittance of the filter, when determined as described in clause 6 of EN167, shall not exceed 0.0032%. Filter transmittance in the waveband 280 to 380 nm (ultraviolet radiation) shall not exceed 0.003% at any wavelength. Transmittance in the near infrared waveband (780 to 1400 nm) shall not exceed 0.027% at any wavelength. Filters with luminous transmittance (in the waveband 380 to 780 nm) equivalent to scale number 12 to 16 as specified in Table 1 of EN169:1992 are considered suitable for direct observation of the sun. It should be noted that many observers will find the solar image uncomfortably bright when filters with scale numbers of 12 or 13 are used.
A filter which transmits 4% of the sunlight is roughly equivalent to a neutral density filter ND 1.4, or welder's glass #4. Definitely not good enough. For comparison, #12 transmits just 0.002% of the light, and #13 is 0.0007%. So looking at Betelgeuse from up close would be very painful, you would not see any details, and gazing for too long will blind you.
A more insidious part of this is the infrared, which we are underestimating because it does not count to the visual magnitude but still damages the retina. The surface temperature of Betelgeuse is about 3500K, compared to 5770K for the Sun, so the ratio of surface flux (intensity over all wavelengths) is that ratio to the 4th power, which is 13.5%. That's like looking at the Sun with Welder's glass #3 or neutral density filter ND 0.9. Don't do it.
I guess this naturally leads to the question -- what filter would you need to safely look at Betelgeuse from up close? Since we're starting at 13.5% of the Sun's surface brightness, we must use a filter that transmits 0.014% of that to be the equivalent of looking at the Sun through a #12 shade. So we need a #10 shade at least to safely look at Betelgeuse!