To view an analog 25kHz signal, you really need something that has a bandwidth much higher than that. And a sample rate a lot higher.
For a digital 4MHz signal, you really will find it useful to have an analog scope. A digital signal analyzer is great as long as the signal is clean, and you are only interested in looking at the data and timing. But you’ll still need something with a capture rate something like 20x the maximum frequency of the clock. At least.
Think about it - a 4MHz signal has a 250ns period. But that is a 125ns time on or off. For a capture rate of 80MHz, that is only 10 samples per ton or toff, or 12.5ns granularity. But what if the duty cycle is not 50%? It gets worse, then.
If the signal isn’t perfect, there may be ringing, or droop, spikes, etc. mucking up the signal. A digital analyzer can’t show you what is going on.
But a 4MHz digital clock is a square wave, which is made up of the fundamental frequency and the odd harmonics in reducing power. The 9th harmonic is a realistic minimum to view a reasonably sharp square wave, ie, 10x analog bandwidth. More is better. But we need to take into account the timing issues discussed above, and duty cycles different than 50% mean different timing -and- higher harmonics necessary to reconstruct the signal.
Can you get away with the $42 DX.com USB scope? Probably 90% of the time, maybe more. But that smaller percentage will soak up an inordinate amount of your time troubleshooting a signal that you can’t see clearly.
I’ve not seen a really good USB scope for $100. For $100, I think I’d look for a good used analog scope with at least a 100MHz bandwidth, dual channel. Or buy that $42 USB scope, and save up about $400 for a decent 1Gsps digital dual or quad trace scope. Don’t waste your money on those little scopes that look like cell phones, they are a waste of money.