These are my pc specs:
OS: Windows 10 Home, 64 bit
RAM: 4 GB
CPU: Intel i3 - 5005u
GPU: Intel HD 5500 Integrated Graphics
Can I run any good modpacks with decent fps?
Crackpack for example?
Most "Good" modpacks, if by "Good" you mean some of the larger ones, will require 4GB (without tweaks) to run well. If you're using Java 1.8 this will be the default setting if you have the memory for it available... If not you will need to set the command switches to 2GB to avoid potential crashes.
Your CPU isn't the best choice for running many games, depending on it's clock speed, but it's usable. I3's aren't the newest chips, but the faster the better. Minecraft doesn't really use multithreading well, so multi-core CPUs don't really help much.
You won't get "decent" fps with most integrated graphics... In addition to being designed to at best display HDMI video, they generally do not have their own RAM and steal what they need from the system reducing what you think you have available. If the integrated GPU needs 256MB, your system only has 3.75GB for example, 512MB reduces system RAM to 3.5GB and so on. Also, with integrated graphics the CPU and GPU have to "take turns" accessing system resources like memory. A separate graphics card will have it's own ram and be able to access what it needs with less impact on the system.
Judging from your specs, this is either an older Desktop or a laptop. If it's a laptop about the only thing you can do, unless you have a deluxe model, is add at least 2GB ram for reasons described above. This will account for what Windows10 requires for itself (1GB) and the RAM stolen from the system by the integrated graphics.
There's also a thread in "Ask a Simple Question" about running larger packs with limited memory by disabling some mods and other tweaks, Look there for info on pack tweaking... I think it's called something like "How to run Infinity Evolved with 1.0GB Allocation"
With a Desktop you have more options. after checking the system specs (ie the Motherboard) you could do the following, in order of preference (and cost, considering Intel prices):
1) Disable integrated graphics and add a GPU card. This frees up some ram (see above) and allows you to choose the GPU YOU want. For example, I'm running a $49 video card right now and seeing 60-70 fps constantly.
2) add 2 to 4 GB ram. This will compensate for Windows10 requirements and give you breathing space for Texture packs or, if you're the type, extra windows open for Teamspeak, Wiki, etc.
3) A CPU upgrade. You might be able to get an I5 (not sure, I use AMD) but even a faster I3 is better.
Hope this helps, or at least informs.