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I actually get

TypeError: unable to make sense of Maxima expression ...

when I try this. In fact,

sage: desolve_rk4(f(t,y), y, ivar=t, ics=[0,-4], end_points=[0,.4])
[[0, -4],
 [0.1, -6.44026308551],
 [0.2, -18.4360863753],
 [0.3, -9638.98851821],
 [0.4, -2.29907718472e+44]]

works, but

sage: desolve_rk4(f(t,y), y, ivar=t, ics=[0,-4], end_points=[0,.5])
TypeError: unable to make sense of Maxima expression '[[0,-4],[0.1,-6.440263085506939],[0.2,-18.436086375306434],[.30000000000000004,-9638.988518207414],[0.4,-229907718471530700000000000000000000000000000.],[0.5,-.000000000000000.000000000000000]]' in Sage

and indeed -.000000000000000.000000000000000 makes no sense inside of Sage. I'm not sure what it's supposed to mean in Maxima, either!

Here is the problem, in Maxima.

(%i1) rk(6+y-y^2,y,-4,[t,0,.5,.1]);
(%o1) [[0, - 4], [0.1, - 6.440263085506939], [0.2, - 18.436086375306434], 
[.30000000000000004, - 9638.988518207414], [0.4, - 2.2990771847153071e+44], 
[0.5, - .000000000000000e+4932]]

Apparently Sage doesn't know how to translate these. And what should zero times ten to the 4932 mean?

Anyway, I've opened Trac 15789 for this.